Eidolon (Final Fantasy XIII)


 * For videos, see: Eidolon (Final Fantasy XIII)/Videos



Eidolons are the summons in Final Fantasy XIII that also appear in one form or another in Final Fantasy XIII-2 and Lightning Returns: Final Fantasy XIII. They are creatures created by the goddess Etro to serve as her emissaries in the visible world and their spirits are locked within crystals known as "eidoliths".

Final Fantasy XIII
 These mystical entities reveal themselves before only to a select few l'Cie. It is said that they are saviors, come to rescue hopeless l'Cie who find themselves bound to a Focus against their will. If this is true, they offer a brand of salvation few indeed would seek willingly: without exception, Eidolons attack the l'Cie whose presences they grace. There have been no l'Cie in Cocoon for centuries, and as a result, no way of determining the truth behind tales of these beings. To the citizens of Cocoon, they remain the stuff of bedtime stories.  Her Providence sought nothing.
 * Her Providence made nothing.

She but looked on, silent in Her sorrow.
 * The Goddess pitied mortals, destined as they were to die, and so She deigned to intervene in the hour of their greatest peril. She averted cataclysm that was to be, and put to rest the ones who would have robbed so many of what time fate had ordained.

Her compassion did not end at this.
 * The Goddess pitied also those subjected to that fate of Focus, crueler still than death. To them She sent Her messengers, to deliver hope when all was lost.

—Sermons of the seeress Paddra Nsu-Yeul

Final Fantasy XIII-2
 Serah's ability to summon monsters is another power born from chaos. The summoner seals defeated creatures in crystals, and call them forth when needed. The weak serve the strong—it is the law of the Unseen Realm that binds the summoned monsters to the will of their master.

Some l'Cie command Eidolons, and the underlying principle is similar. The difference that sets this new power apart, however, lies in the ability to acquire multiple monstrous allies. In her dream, Serah saw Lightning leading an entire army of beasts, and this is likely a more powerful version of the same ability.

Final Fantasy XIII-2
 Eidolons are beasts that dwell in Valhalla. L'Cie chosen by Etro are pitted against Eidolons in a trial by combat. Those who emerge triumphant receive a crystal called 'eidolith,' which they may use to summon the beast to fight at their side.

The weak must obey the strong. This is the inviolable law of Valhalla. Like any other denizen of the Unseen Realm, Eidolons are bound to answer every call of their master.

Profile
The Analects on the goddess Etro suggest that Eidolons are created by her and are her messengers, "to deliver hope when all was lost". The Eidolons' connection to Etro is shown by having her script written across their bodies and during their summon animations. The Eidolons' nature as mechanical beings confers with the similar beings of fal'Cie, as well as the gods of Final Fantasy XIII universe, of which at least Pulse and Bhunivelze are shown to be mechanical in appearance as well.

As revealed in Final Fantasy XIII-2 and Final Fantasy XIII-2 Fragments After, various Eidolons reside in Valhalla, the middle world between the world of the living and the unseen realm of the dead. They follow Odin, as the rules established in Valhalla dictate that they must obey those stronger than themselves. As such, when an Eidolon manifests in response to a l'Cie on the verge of despair, it forces the human to fight it. Once defeated, an Eidolon becomes a loyal follower to the l'Cie and provides an eidolith, a uniquely shaped crystal that enables the Eidolon to be summoned by its master. Fragments After states that Eidolons communicate with their masters via telepathy.

Though normally summoned by a l'Cie, the people of Paddra devised a means to summon Eidolons via incarnate summoning, where a l'Cie summons their Eidolon, but instead of manifesting it as a partner in battle, they manifest it within their own body, causing the two to permanently fuse. The resulting being is drastically more powerful and can shift between human and Eidolon forms at will. The only known user of this form of summon is Caius Ballad who used it to fuse with Bahamut. Incarnate Summoning is rare, as using it is at the cost of the summoner's life; Caius is an exception, as he was saved by goddess Etro who gave him the "Heart of Chaos" to make him immortal.

Final Fantasy XIII
When Lightning and her companions are branded l'Cie, Etro sends an Eidolon to each of them when they falter on their path. Snow earns his Eidolons first, soon after the party's branding, and eventually Lightning, Sazh, Fang, Hope and Vanille follow. Upon entering Gran Pulse from Cocoon for the first time the party's airship is destroyed by a wyvern, but Lightning helps Fang summon her Eidolon, Bahamut, and the party rides it down to the surface. When the party returns to Cocoon they inadvertently crash into a grand prix, and ride it out with their Eidolons in Gestalt Mode. After Ragnarok crystallizes Cocoon the party enters crystal stasis, and upon being released from it by Etro, they find their l'Cie brands wiped clean with their ability to summon Eidolons no longer available.

Final Fantasy XIII-2
Soon after Lightning is dragged into Valhalla by chaos, prior to becoming Etro's champion, Lightning encounters Odin and defeats him to renew their partnership while forging a new one with Bahamut. With an army of Eidolons at her service, Lightning orders them to battle Caius Ballad's army of Rift Beasts while riding Odin during her initial battle with Caius. She summons Bahamut to save a falling Noel Kreiss, before sending him away to New Bodhum 3 AF to find her sister, Serah Farron. Odin stays by Lightning's side till the end, and she sends the steed to aid Noel and Serah once the two find their way to Valhalla to defeat Caius once and for all.

Lightning Returns: Final Fantasy XIII
After Caius's scheme to kill Etro succeeds, Lightning awakens in Nova Chrysalia 500 years later and finds herself unable to summon Odin. Odin endures in the form of a white chocobo known as the Angel of Valhalla that Lightning nurses back to health after saving the injured bird from a Chocobo Eater prior to learning of Odin's condition from Yeul. Caius retains the ability to summon Bahamut, though he does not perform an incarnate summoning. The Eidolons aid their former allies in the final assault on the god Bhunivelze, and after the end of time and the beginning of a new, god-free universe, the Eidolons disappear to reincarnate, bidding farewell to their former masters.

Acquiring
Usually appearing when a l'Cie is in deep despair, an Eidolon arises to battle its l'Cie, casting Doom on its respective summoner at the battle's start. The goal is not to reduce its HP to zero, but to fill its Gestalt Gauge within the allotted time.

Even if the Eidolon's Libra page lists different methods for raising the Eidolon's Gestalt Gauge depending on the Eidolon (namely, the actions available from the party leader's default paradigm roles), all of the Eidolons' Gestalt Gauges do, in fact, raise by exactly the same measures. Therefore, using the party leader as a Synergist will boost any Eidolon's Gestalt Gauge, even if it is mentioned only for Brynhildr and Alexander (because Synergist is one of Sazh and Hope's primary roles). However, as the Eidolons are battled early enough in the game for the characters to unlikely have explored Crystarium trees other than their primary roles, this factor is insignificant.

Summon multipliers:
 * Methods to raise the gauge

The party leader's role is the most important factor when it comes to calculating the Gestalt Gauge increase. The increase in Gestalt Gauge is calculated as percentages.

As for all allies all they can do to contribute toward the Gestalt Gauge fill is to raise the enemy's chain bonus with command abilities. The allies' Gestalt Gauge Increase % is calculated as: Chain Bonus increase * 0.6 * Summon Multi.

The Sentinel role increases the Gestalt Gauge by being attacked by the Eidolon while the Sentinel using its counterattack and defensive commands. The following chart illustrates by how much the gauge fills when the Eidolon attacks a party leader who is using a Sentinel ability. Because the party leader in battles against Odin and Brynhildr can't be a Sentinel, they are not included in the below chart.

Shiva


Stiria and Nix are faced by Snow in Lake Bresha. The battle commences after the the point of view returns from Lightning's party to Snow, who has remained behind to dig Serah's crystal out of the crystallized lake. He is apprehended by PSICOM, and when surrounded inadvertently summons Shiva. After the Shiva Sisters have annihilated the PSICOM forces, the sisters turn against Snow, who must defeat them to tame them.

Since Snow fights his Eidolons alone, Stiria cannot be targeted during the fight, but instead heals Snow as he has no other support. Shiva Sisters "yield to those who defend against attacks", so the simplest method of victory is to Paradigm Shift to Sentinel, and repeatedly use Steelguard.

Odin


Odin is fought by Lightning and Hope in the Vile Peaks. The battle commences in the Scavenger's Trail area of the Vile Peaks when crossing a bridge. Hope stumbles and Lightning loses her patience with him, unknowingly summoning Odin, who turns against Hope, and the two battle the Eidolon together.

Odin is an aggressive Eidolon, initially targeting Hope with physical attacks and Thundara. Lightning can satisfy Odin by building chain bonuses and healing, and Lightning will need to do both as Odin's strikes will necessitate healing greater than Hope alone can supply. Odin will often guard with Ullr's Shield, allowing for either a break to heal or an uninterrupted attacking phase.

Brynhildr


Brynhildr is fought by Sazh and Vanille in Nautilus Park The Fiendlord's Keep area, where Brynhildr manifests as a result of Sazh and Vanille's confrontation escalating. Brynhildr attacks with fire attacks that hit both Sazh and Vanille. The key to winning the battle is employing Sazh as a Synergist and only switching to offensive roles when all buffs he has available are in place.

Bahamut


Bahamut is fought by Fang, Lightning and Vanille in the Fifth Ark's final area called The Apse, where Fang turns against the party when they plan to defy their Focus, inadvertently summoning Bahamut. Bahamut is an aggressive Eidolon, initially targeting Fang with physical attacks followed by Whirlwind on the party. Fang can appease Bahamut by fighting as a Saboteur and switching to Sentinel to guard against Bahamut's strongest attacks.

Alexander


Alexander is fought by Hope, Lightning, and Fang in the Vallis Media Fingers of Stone area, after finding Hope passed out at the north end and bringing him back to the camp. As Hope loses his drive to continue and asks the others to go on without him, he unwittingly summons Alexander, who turns against him, but Lightning and Fang rush to his aid. Hope can win against Alexander by acting as a Synergist and healing the party when they are on low HP, and acting as a Ravager during times no support is needed.

Hecatoncheir


Hecatoncheir is fought by Vanille and Fang in the Mah'habara Subterra's Flower-filled Fissure. The battle commences after entering the area when Vanille and Fang talk about their role in the War of Transgression and Fang discovers Vanille has been lying to her ever since they woke up from crystal stasis. Vanille's emotional turmoil calls forth her Eidolon, that attacks the pair. Fang should remain as a Sentinel for most of the battle leaving Vanille to debuff as Saboteur and heal and use Ravager when neither are needed. When Hecatoncheir is charging, the party can use offensive paradigms, but Fang should switch to Sentinel when he starts attacking.

Final Fantasy XIII-2
In Final Fantasy XIII-2 the player characters are not l'Cie and thus do not obtain Eidolons, but "dark" versions of Odin and Bahamut are battled as bosses.

Twilight Odin


Twilight Odin is fought twice by Serah and Noel in the Vile Peaks, once in 10 AF and and once in 200 AF. The battle commences after approaching a cloud of chaos present in the area. The key to defeating him is staggering him quickly. In the first battle Twilight is susceptible to status ailments after being staggered, but in the second battle debuffs hit him since the start.

Once Twilight Odin is defeated in the Vile Peaks 200 AF he will join the player party as a Commando.

Chaos Bahamut
Chaos Bahamut is fought twice: once as part of the main storyline and once as part of the pregame tutorial. Chaos Bahamut is also fought in the DLC episode "Lightning's Story: Requiem of the Goddesss". The tutorial battle is made easy, but Chaos Bahamut may prove a formidable opponent in his other boss appearances. Chaos Bahamut is fought at the end of the Academia 500 AF scenario where he has two forms. At first he simply hovers before using Metamorphosis, upon which he becomes a flying mechanical wyvern. In its normal form, it will utilize physical attacks in sets of two, and after changing between forms he uses magic attacks, such as Megaflare. The best way to counter Chaos Bahamut's onslaught of powerful attacks is to deploy Sentinels.

Lightning rides Odin in the tutorial battle in the prologue, and can use abilities reminiscent of Odin's summon abilities from Final Fantasy XIII: Ullr's Shield and Lightning Strike. Despite the abilities sharing names they are not the same as the abilities in Final Fantasy XIII, as Ullr's Shield is now a healing skill and Lightning Strike is more akin to a regular Thunder spell.

Summoning
Eidolons are summoned by spending three Technical Points and fight alongside the summoning party member in battle while the other members temporarily leave. At this time no role bonuses are contributed by the allies who have left the battle. When summoned into battle, the summoner is temporarily invincible, making it a strategic asset; if summoned right when the opponent is about to launch their attack, the party can summon an Eidolon and survive, as the summoner is temporarily invincible during the summoning animation.

The time at which the Eidolon can stay is determined by its SP gauge, which decreases as time passes and also decreases whenever the Eidolon receives damage. The summoner has a Gestalt Gauge, which increases in the same way as how it increased when the player battled against that Eidolon. The gauge is quickly drained when the Eidolon runs out of SP. When both the SP and the Gestalt Gauges are reduced to zero, or if the summoner is knocked out, the Eidolon is dismissed but it will use Arise spell before it leaves, fully healing the party.

One can activate "Gestalt Mode", to have the Eidolon change into a form the player can ride, shifting the battle to a more action-oriented tone in which button inputs correspond to attacks. Gestalt Mode's duration is determined by how high its Gestalt Gauge was when Gestalt Mode was initiated, and each attack subtracts a certain number of points from the gauge. It allows access to the Eidolon's finisher, which consumes all the points left in the gauge. Eidolons grow in strength as their summoner grows. Each Eidolon has a concept that defines their actions in battle.

The finisher has three levels, LV1 finisher being the weakest, and LV3 finisher being the strongest. Spending more units when initiating the finisher doesn't affect its strength, only the level does. The finisher's level is determined by how much the player managed to increase the Gestalt Gauge during the time the Eidolon is summoned. Also, using the finisher resets the chain gauge.

When an Eidolon is dismissed, regardless of whether it entered Gestalt Mode or not, all party members regain maximum health and K.O. characters regain consciousness making summoning useful as a last resort.

Rank
The Eidolon's rank determines its stats when summoned to battle. The rank is the sum of the character's Crystarium level and the number of level 5 roles. Thus the maximum rank is 16 (Crystarium level 10 + all roles at level 5).

SP
The Eidolons are given a base 50 seconds from the moment of summoning. The time they can stay is displayed as the SP gauge. The full SP gauge is equal to 50 seconds no matter how much SP the Eidolon actually has. When summoned to battle the Eidolons don't have an HP gauge, instead, any sustained damage is depleted from the SP gauge reducing the time the summon can stay and battle. If an Eidolon with 10,000 SP takes 1000 damage, it has lost 1/10th of its max SP, or 5 seconds of time. If the Eidolon has 50,000 SP, it has only lost 1/50th of its max SP or just 1 second.

Gestalt Gauge
The Gestalt Gauge starts half full. The method to raise the gauge is the same as the method to raise it during the battle against the same Eidolon, except the summon multipliers are ignored. The Gestalt Gauge determines the number of moves the summoner can perform in Gestalt Mode, as well as the level of the Eidolon's finishing attack. If the Eidolon's SP gauge completely depletes, the Gestalt Gauge will begin to deplete at a rate of 20% each second.

Shiva
The Shiva sisters, Stiria and Nix, are Snow's Eidolons, obtained in Lake Bresha. They have their own separate ATB gauge, but both will be dismissed if their shared SP falls to zero.

In normal mode, Stiria focuses on the Medic and Ravager roles, healing Snow and dealing Ice-elemental damage on the enemies. Nix focuses on a Commando role, as her abilities are physical based, although she has Blizzara at her disposal. While Stiria cannot be targeted by enemies, she can still be damaged by wide area attacks.

Snow's Gestalt Gauge increases by driving up enemy chain gauges and by defending against enemy attacks. In Gestalt mode, the Shiva sisters transform into a motorcycle. Their concept is Rapid Blaster. While in Gestalt Mode only Stiria's stats are used in the damage calculations.

A different variation of the Shiva Sisters return in Final Fantasy XIII-2, supporting Lightning in her fight against Caius to protect Valhalla.

Stats
Stiria has 3 ATB bars and Nix has 4.

Stiria's abilities
Stiria attacks the enemy Snow is targeting with Blizzard and does nothing while its ATB gauge charges. If Snow is afflicted with a negative status it casts Esuna. If Snow's HP is below 70% it casts Curaga.

Nix's abilities
Nix attacks the enemy Snow is targeting with Wheel Rap three times followed by Flip Kick. While the ATB gauge charges it closes in on the enemy it is targeting and ATB Charge accelerates the charge rate.

Gestalt Mode
The Latin writing on the sphere of ice during the summoning animation reads:
 * By the grace of Etro, twin rulers arise from your icy thrones. Come forth, sisters of Winter wastes.
 * A name in blood, a pact of ice. Shiva shall rise, her bond eternal and unyielding.
 * Aurora Borealis
 * Aurora Australis

Odin
Odin is Lightning's Eidolon, obtained in the Vile Peaks. He is lightning-elemental and has many powerful and diverse physical and magical attacks. He acts as a Sentinel, drawing all enemy attacks. As Odin struggles driving up enemy chain gauges, it may be better to place Lightning in the role of Ravager to increase Odin's Gestalt Gauge.

Lightning's Gestalt Gauge increases by driving up enemy chain gauges and by healing herself and removing status ailments. In Gestalt mode, Odin transforms into the horse Sleipnir, named after Odin's horse in Norse mythology. His concept is Multi-Attacker. Odin's sword is known as Zantetsuken and splits in two for Lightning to wield while riding Sleipnir.

In Final Fantasy XIII-2, Odin still serves as Lightning's Eidolon and aids her in fighting Chaos Bahamut. Some of the abilities used by Lightning are named after the abilities from Final Fantasy XIII, but they are not the same abilities. These are Lightning Strike and Ullr's Shield.

Four other Odins charge with Lightning in Valhalla when she fights Caius. A corrupted variation of Odin, named Twilight Odin, appears in the Vile Peaks in the years 10 and 200 AF as a physical manifestation of chaos, using the memories of Lightning's summoning in the area years ago as a source of energy. He is fought as a boss in both areas and can be recruited as a Paradigm Pack monster for the party. Twilight Odin is also a rare drop of the Level 99 Valfodr, this being the only other way of obtaining his monster crystal, aside from defeating him in the Vile Peaks in 200 AF.

In Lightning Returns: Final Fantasy XIII, what happened to Odin is revealed after doing the main quest in Temple of the Goddess. Odin also appears to help Lightning at the end when she calls out for help. In the Lightning Returns: Final Fantasy XIII Ultimania, director Motomu Toriyama referred to Odin as a "good friend" to Lightning, stating that his inclusion in the game was as someone Lightning could open her heart to.

Stats
Odin has 5 ATB bars.

Abilities
Odin attacks the enemy Lightning is targeting with Flourish of Steel, Skyward Swing, and Seismic Strike. It will stock four attacks from the aforementioned abilities on its ATB bar and then have Thundaga as the final attack. If the enemy is far away it will use Crushing Blow. When Odin uses Thundaga it takes only one ATB slot, compared to when player characters use it when it takes three ATB slots. Odin works as a Sentinel in battle drawing in all enemy attacks with its Valhalla's Call.

Gestalt Mode
The Latin writing on the symbol that emerges from the ground when Lightning summons Odin reads:
 * By grace of Etro, let thunder herald your arrival. Come forth, sunderer of falsehood.
 * A name in blood, a pact of truth. Odin shall rise his bond eternal and unyielding.

The plate on Odin's back (as a horse) reads:
 * My weapon is light, my steed is thunder. I am the herald of truth. I am Odin.

Odin's shield that becomes the horse's shoulder plate, says:
 * Keeper of truth. Destroyer of corruption. Odin.

Brynhildr
Brynhildr is Sazh's Eidolon, obtained in Nautilus Park. By default, Brynhildr is Fire-elemental, but Sazh can bestow En-spells on Brynhildr allowing her to change her elemental characteristics. Brynhildr is strong physically, but also has a wide array of spells based on which enhancements Sazh has cast on her.

Sazh's Gestalt Gauge increases by driving up enemy chain gauges and by augmenting himself and Brynhildr with status enhancements. In Gestalt mode, Brynhildr transforms into a race car with side mounted machine guns. Her concept is Gadget Master.

Brynhildr does not make an appearance in Final Fantasy XIII-2, but concept artwork for a blue variation has been shown in the Final Fantasy XIII-2 Ultimania Omega, suggesting it was once considered as one of the Eidolons seen in Valhalla.

Stats
Brynhildr has 4 ATB slots.

Abilities
Brynhildr uses Slash twice followed up with Valkyrian Scythe and finishes with Pyroburst. She targets the same enemy Sazh is targeting. If ATB Boost activated the previous turn, Brynhildr will follow up Pyroburst with another Valkyrian Scythe followed by another Pyroburst. While Brynhildr's ATB gauge is filling, she moves toward the enemy.

Gestalt Mode
The text on on the symbol that appears in the ball of fire reads:
 * By the grace of Etro, warrior maiden be drawn by the fires of battle. Come forth, wreathed in blazing glory.
 * A name in blood, a pact of flame. Brynhildr shall rise, her bond eternal and unyielding.
 * Rousing Inferno.
 * Ethereal Blaze.

Bahamut
Bahamut is Fang's Eidolon, obtained in the Fifth Ark. Bahamut utilizes powerful non-elemental attacks, making him the Eidolon with the highest damage-dealing potential. As Bahamut's SP is rather low compared to other Eidolons, it may be a good idea for Fang to assume the role of Sentinel to increase the summoning time as well as to boost Bahamut's Gestalt Gauge whenever Fang is attacked while blocking.

Fang's Gestalt Gauge increases by driving up enemy chain gauges, defending against attacks and inflicting status ailments upon enemies. In Gestalt mode, Bahamut allows Fang to ride him. His concept is Aerial Raver.

In Final Fantasy XIII-2, while Lightning is able to summon Bahamut, Caius can transform into a variation of Bahamut called "Chaos Bahamut" who is fought at numerous points throughout the game.

Stats
Bahamut has 5 ATB slots.

Abilities
Bahamut targets the same enemy as Fang and uses Dragon Claws three times followed by Whirlwind and finishes with Inferno. Bahamut retreats from the enemy while waiting for its ATB gauge to fill. If the enemy is already far Bahamut will do nothing while charging its ATB gauge.

Gestalt Mode
The text in Bahamut's summon animation reads:
 * By the grace of Etro emerge from the midnight void. Come forth, master of the rift.
 * A name in blood, a pact of ruin. Bahamut shall rise, his bond eternal and unyielding.
 * Shoulder the sky. Enfold the earth.

Alexander
Alexander is Hope's Eidolon, obtained in Vallis Media. Like Bahamut, Alexande utilizes strong non-elemental attacks, but they are all physical. Alexander draws in all enemy attacks and has the highest starting SP of all Eidolons, but he is slow moving around the battlefield and takes longer to act than the other Eidolons, in some cases being late in healing Hope.

Hope's Gestalt Gauge increases by driving up enemy chain gauges, bestowing status enhancements, removing status ailments, and restoring HP. In Gestalt mode, Alexander transforms into an immobile fortress hugging one edge of the battlefield. Whenever Alexander is summoned, executing gestalt or unleashing his final gestalt attack, a somewhat faint, angelic organ pipe can be heard. His concept is Tactical Commander.

Alexander does not make an appearance in Final Fantasy XIII-2, but concept art for it exists, and is included in the Final Fantasy XIII-2 Ultimania Omega guide book.

Stats
Alexander has 4 ATB slots.

Abilities
If there is at least one enemy who is not provoked and can be provoked, Alexander uses Lofty Challenge. Otherwise, Alexander targets the same enemy as Hope and uses Steelcrusher twice followed up with Soaring Uppercut finishing with Blast Punch. If the enemy can be launched it uses Soaring Uppercut. If the enemy is far away it uses Blast Punch and if the enemy is surrounded by other enemies it uses Explosive Fist.

Gestalt Mode
Alexander is the only Eidolon who has attacks in Gestalt Mode that ignore type-based defense.

The writing on Alexander and his summon symbol read:
 * By the grace of Etro, stand fast your walls of stern judgment. Come forth, fortress of sanctity.
 * Fortified virtue. Shattered iniquity.
 * A name in blood, a pact of light. Alexander shall rise, his bond eternal and unyielding.

Hecatoncheir
Hecatoncheir is Vanille's Eidolon, obtained in Mah'habara Subterra. Hecatoncheir is Earth-elemental and is the only Eidolon that possesses the Quake ability. He is also the only Eidolon that doesn't have Curaga, meaning Vanille must heal for herself.

Vanille's Gestalt Gauge increases by driving up enemy chain gauges, inflicting and removing status ailments and restoring HP. In Gestalt Mode, Hecatoncheir transforms into a bipedal mech with four machine gun turrets. His concept is Shooting Blaster.

Stats
Hecatoncheir has 6 ATB slots.

Gestalt Mode
The writing on Hecatoncheir and his summon symbol reads:
 * By the grace of Etro, arise great and mighty giant. Come forth, thee who shoulders the sky.
 * A name in blood, a pact of strength. Hecatoncheir shall rise, his bond eternal and unyielding.
 * Impure hands purge arms that fortify weakness.

Other Eidolons
During the Pompa Sancta parade at Nautilus a number of Eidolons appears to enact a version of the War of Transgression, of the Sanctum l'Cie defeating Ragnarok. Before the parade Sazh comments, "They're putting Eidolons on a parade".

These Eidolons are vastly different from the ones the party can summon. They appear organic rather than mechanical, and appear to be only special effect visuals generated for the purpose of the parade that bear allusion to the series' past summons. The Eidolons seen at the parade are Carbuncle, Siren, Ramuh, Ifrit, and Valefor. Although Leviathan doesn't make an appearance, concept artwork for it exists, indicating Leviathan was considered as one of the Eidolons to appear in Final Fantasy XIII.

Development
The way summon sequences are utilized in Final Fantasy XIII stemmed from players of older Final Fantasy games not being keen on waiting for the summoning sequence amid a battle to finish to see the damage done. Thus things were mixed up in Final Fantasy XIII: the summon sequences still have flashy effects, but the summoning scenes are short and paired with gameplay. A dummy/placeholder model was used for testing the summons, the model being similar to the Test 0 enemies of Final Fantasy VII, consisting of a simple, untextured double/sided pyramid.

The idea of transformation was a theme the designers had for Final Fantasy XIII; that's why the Eidolons have two forms, and many of the characters' weapons transform between two forms as well, such as Lightning's gunblade and Sazh's pistols. Final Fantasy XIII Ultimania Omega reveals some ideas the designers had behind the Eidolons. It had been decided that Odin was going to be a horse in the planning stage, and in his normal mode he was made to look like the Joker from a deck of cards. Odin was originally more harsh looking and dark colored, but the developers wanted Lightning to be a knight with a white horse. Since Lightning had lost her parents at a young age, and tried to take their place and raise Serah, Odin was imagined as someone she would project a fatherly image onto. Snow's Shiva Sisters represent the Farron sisters, Lightning and Serah.

The Final Fantasy XIII-2 Ultimania Omega reveals scrapped plans for an extension to Snow's DLC scenario episode titled "Perpetual Battlefield". Alongside the existing part of the episode in the Coliseum, there would have a separate minigame in which players would play as Snow as he rode through the Archylte Steppe on the Shiva Sisters. The player's objective would have been to try and kill 30 monsters using Snow's skillset—which included being able to spin, drift, jump and use a water-based spell. However, once this part of the DLC was scrapped, this minigame was used as a base for the chocobo racing minigame in Serendipity.

Musical themes
When fighting Eidolons, there are three possible battle themes that will accompany the fight. "Eidolons" plays during Snow's battle with the Shiva Sisters, Fang's battle with Bahamut, and Vanille's battle with Hecatoncheir. "Test of the L'Cie" plays during Sazh's battle with Brynhildr and Hope's battle with Alexander. Lightning's battle with Odin is accompanied by "Desperate Struggle".

Final Fantasy XIV
Odin and Bahamut appear in their Final Fantasy XIII incarnations as part of a series of FATEs in Final Fantasy XIV linked to Final Fantasy XIII and featuring Lightning as a non-playable battle ally. Bahamut is one of the enemies faced in the FATEs, while Odin appears once more as a summon Lightning can use in battle.

Dissidia 012 Final Fantasy
Brynhildr and Hecatoncheir appear as summons.

Lightning's EX Burst is Gestalt Drive, but she does not ride Odin, although she uses the same attacks when summoning Odin in Gestalt Mode in Final Fantasy XIII: Lightning Strike, Thunderfall, Stormblade, Razor Gale and Zantetsuken.

Final Fantasy Airborne Brigade
Some of the Eidolons appear in Final Fantasy Airborne Brigade.

Final Fantasy Record Keeper
Some of the Eidolons from Final Fantasy XIII appear as enemies in Final Fantasy Record Keeper.

Final Fantasy Trading Card Game
The Eidolons appear as cards in this trading card game, representing the same elements they do in Final Fantasy XIII. Bahamut, that is non-elemental in Final Fantasy XIII, is depicted in the Fire set.

Triple Triad
The Eidolons appear on Triple Triad cards in the version available via Final Fantasy Portal App.

Merchandise
T-shirts with Eidolon designs have been on sale in the Japanese Square Enix store. The shirts cost ¥3,900, with two versions, Odin or Shiva. The Eidolons have also been made into action figures in the Final Fantasy Creatures Kai Vol. 3 series. These figures cost ¥6,240 a piece. Final Fantasy XIII -Play Arts Kai- figure of Odin was released in July 2010; it has two forms, Sleipnir or Odin. Final Fantasy XIII -Play Arts Kai- Shiva Bike Action Figure Set with Nix and Stiria was released in November 2010 with the Shiva Sisters, which could either be assembled separately or into a bike as they appear in Gestalt Mode.

Etymology
This alludes to the Eidolon and the l'Cie combining their powers when the latter rides the former.

Trivia

 * The l'Cie hailing from Pulse and Cocoon have different means of summoning their Eidolons, despite all being Pulse l'Cie. Lightning, Sazh, Snow, and Hope all destroy their crystals in some way either with their weapons, or, in Hope's case, a magical explosion, while Vanille and Fang's crystals simply disappear into the ground and sky, respectively.
 * Lightning and Snow share a quote when entering Gestalt mode with their respective Eidolons: "We do this together... Let's ride!"
 * Hope and Vanille share a quote when exiting Gestalt mode with their respective Eidolon's finishers: "Ah... see you next time!"
 * When all party members are counted, along with their Eidolons (counting both Shiva Sisters), there are thirteen members of the party. Final Fantasy XIII contains many allusions to the number thirteen.
 * Odin and Bahamut are the only two Eidolons that have their Gestalt Mode in animal/beast forms, while others looks like vehicles, or weapons.
 * Water is the only element not portrayed by an Eidolon.
 * Eidolons can naturally break the damage limit even if a character does not have a Genji Glove accessory equipped, allowing the player to earn the Limit Breaker Trophy/Achievement without one.
 * A researcher in Yaschas Massif 01X AF in Final Fantasy XIII-2 mentions that records of magic and sorcery were found in the Paddra Ruins, including a spell to summon Eidolons. It was a forbidden art, as it shortened the conjurer's lifespan.
 * Bahamut has a similar entrance as the one in Final Fantasy X, dashing from the sky through his summoning emblem, and making a soft landing as they are being summoned.
 * Brynhildr appears with Noel in the finale of Lightning Returns: Final Fantasy XIII, despite him never being a l'Cie, as Sazh is not present.