Soul (Fabula Nova Crystallis)

Souls, also referred to as Hearts, are an integral part of the Fabula Nova Crystallis: Final Fantasy subseries. Gifted to humanity by the Goddess Etro, they act as a balancing force in the universe, and are thus key to the plans of the mythos's deities and the fal'Cie they create to aid them. Souls are the means through which humans have the strength to challenge both the fal'Cie and the deities.

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When humans were created by Lindzei from Etro's blood after her suicide, they were soulless. After her death Etro moved on to the Unseen Realm and was entrusted with keeping the balance between the Visible Realm (the world of the living) and the Unseen Realm (the world of the dead) by the creator goddess Mwynn, who was soon consumed by a substance known as chaos. Etro felt an affinity with humanity who are bound to follow her in death, and as the humans passed on to the Unseen Realm, Etro gifted them with pieces of chaos making them a key factor in sustaining the balance of the universe. As humanity's souls, made from chaos, pass into the Unseen Realm when the body dies, they are eventually returned to the Visible Realm in a new form when new life is being born, creating a cycle to keep the level of chaos constant. Those who can harness their inner chaos can use magic without being made l'Cie, use Time Gates and control monsters. Furthermore, memories play a part in the cycle as the memories of the living maintain the identities of the departed. Should a deceased be completely destroyed, all memory of them is erased from the living's minds.

As stated in Lightning Returns: Final Fantasy XIII, the deity Bhunivelze, and likely his creations Pulse and Lindzei, cannot see souls because of their links to chaos. Bhunivelze despises it, seeing chaos and the souls residing within it as a polluting force.

The appearance of souls across the games in the Fabula Nova Crystallis series varies, though they do have one constant form: in all the released games they have appeared as glimmering crystals floating in the air. In Final Fantasy XIII the souls of humanity appear like this in preparation for journeying through Etro's Gate. They also appear in this form in Final Fantasy XIII-2, although they are not mentioned in much detail and only play a cursory role in the story. In Lightning Returns: Final Fantasy XIII souls also appear as glowing lights. Souls that failed to "make it" to the other side get trapped in the Chaos and sometimes emerge from Chaos infusions as soul seeds. The Order of Salvation buys the seeds from merchants, who enlist seed hunters to find the soul seeds from within the Chaos. The Order is said to experiment on the souls, but Lightning theorizes Bhunivelze is using them in the Ultimate Lair to create a superior species to replace mankind with. Other alchemists and forges have also done experimenting of their own with the seeds with disastrous results.

In Final Fantasy Type-0, souls appear to be synonymous with the substance known as Phantoma. It appears in various colors, but always as a smoke-like substance that flows out of enemies when they are killed. Phantoma can be given willingly for a cause, as seen when the l'Cie Caetuna uses the voluntarily-surrendered Phantoma of hundreds of soldiers from Rubrum to summon the Grand Eidolon Alexander. Souls in their crystalline form appear when souls and memories of Class Zero speak to Arecia Al-Rashia.

In Final Fantasy XV, a soul is similar in form to those in Final Fantasy XIII, although it is unknown whether they vary in appearance or not.

Final Fantasy XIII
Cocoon's fal'Cie, the creations of Lindzei, concoct a plan to summon their Maker from the Unseen Realm by sending a flurry of souls through Etro's Gate, hoping to "throw it wide open." The fal'Cie plan to use Pulse l'Cie with the Focus to become the monster Ragnarok to destroy the fal'Cie that sustains Cocoon, Orphan, and send the planetoid crashing onto Gran Pulse exacting genocide on mankind. The first attempt, during the War of Transgression, was foiled by Etro when she sent the chosen l'Cie, Oerba Dia Vanille and Oerba Yun Fang, into premature crystal stasis.

For the second attempt, the Pulse fal'Cie Anima awakens Fang and Vanille and brands Lightning, Snow Villiers, Serah Farron, Hope Estheim and Sazh Katzroy as l'Cie bearing the Focus to transform into Ragnarok. During the Siege of Eden the party witness incorporeal crystals glimmering the air, signifying the souls of humans killed in the conflict, and the opening of Etro's Gate on the skies of Eden. After Orphan's destruction Fang and Vanille become Ragnarok and build a crystal pillar to halt Cocoon's fall, stalling the fal'Cie's plans.

Final Fantasy XIII-2
Caius Ballad plans to force open Etro's Gate and unleash Chaos into the mortal world to create a timeless realm to save Paddra Nsu-Yeul, the first human created from Etro's blood who was unable to fade into the sea of Chaos on Valhalla like the other souls. Etro, taking pity on her, put her in an endless cycle of reincarnation, which splintered Yeul's soul and created a source of Chaos in the Visible Realm. This imbalance between the realms causes time distortions and leads to the events of Final Fantasy XIII-2 after Etro reached into the Visible Realm to free Lightning and her friends from their l'Cie bonds.

At first it seems Caius's scheme is to destroy the old crystallized Cocoon and the new man-made Cocoon in 500 AF to trigger a flood of souls through Etro's Gate, but his true goal manifests when Noel Kreiss destroys the Heart of Chaos, the source of Caius's immortality and Etro's life. With Etro dead the balance between the realms is broken and the unrestrained Chaos of Valhalla bleeds into Gran Pulse.

Lightning Returns: Final Fantasy XIII
With Etro dead and the barrier between the realms of the living and dead eroded fusing the two into a new world known as Nova Chrysalia, people's souls can no longer pass on. People hold onto their souls as they have become biologically immortal, yet unable to bear children, and mankind stagnates into a perpetual existence that lasts centuries.

Bhunivelze chooses Lightning as the savior tasked to rescue the souls of Nova Chrysalia in preparation for their life in a new world: in reality, Bhunivelze wishes to purge the rescued souls of their darkness while destroying the souls of the dead, revealed to be able to continue existing because of the living's awareness of them. Lightning saves souls by freeing them of their burdens in varying ways, such as fighting and defeating certain people or performing a task which had been weighing on a person's mind. In the case of Dajh Katzroy, she must seek out fragments of his soul to rouse him from his sleep. There are some she is unable to save, however, such as Caius, who has become bound to the Chaos of the world.

On Nova Chrysalia's final day Luxerion is engulfed in Chaos and the souls glimmer in the air of the cathedral as fine crystal dust, Lumina playing with a crystalline soul shard when she tells Lightning of the link between souls and memories. Lightning disrupts the Soulsong — ritual meant to destroy the souls of the dead — and Vanille, who has gained the power to hear them, frees the souls in preparation for their rebirth. Bhunivelze prepares to purge the souls himself, but Lightning halts his plans. After they are freed from his control, the souls of humanity unite and deliver a final blow to Bhunivelze. All but one incarnation of Yeul stay to maintain the balance between worlds in Etro's stead and the souls of humanity, along with Lightning and her friends, find a new home free of gods and Chaos.

Final Fantasy Type-0 and Final Fantasy Agito
The souls of humans form the core of Arecia Al-Rashia and Gala's plan to open the Unseen Gate, somehow altering the cycle so that the living would be unable to remember their dead. While Arecia tries to use twelve chosen souls of exceptional strength to find and open the Gate, Gala uses his Lulusath Army to slaughter the people of Orience, wishing the flood of souls would force the door open. Both failed each time and the events repeated endlessly, as the land is locked in a stable time loop for the purposes of the experiment. The cycle is repeated 600,104,971 times but the final time, Class Zero, the people chosen to try and force open Etro's Gate, alter the course of events and bring an end to the cycle. After their deaths, Arecia listens to their souls and frees Machina and Rem, two discarded members of Class Zero, from crystal stasis. She allows the people of Orience to remember their dead before finally abandoning the experiment.

Final Fantasy XV
According to a legend from the country of Tenebrae, whenever Etro opens the Unseen Gate the souls of the dead ascend as a streak of light. Those who have a near-death experience see the light and receive magical powers from the Unseen Realm. Both Noctis Lucis Caelum and Stella Nox Fleuret bear magical powers, although further details are unknown.

Trivia

 * In Lightning Returns: Final Fantasy XIII, souls take on a variety of forms similar to previous substances or phenomena related to the dead. The souls as rescued by the savior resemble the pyreflies of Final Fantasy X, and during the ending the souls of humanity take on a form similar to the Lifestream as it appears in Final Fantasy VII: Advent Children.
 * The glimmering crystal shards also appear within the Pulse Vestige in Final Fantasy XIII during the party's trek inside to save Serah, but no one comments on it like they do in Eden. It is unknown whether the crystals here are meant to signify the departing souls (such as from the ongoing Purge down below) or not.