Somnus Lucis Caelum

Somnus Lucis Caelum, more commonly known as the Founder King (as the historical person) and as the Mystic (as the Lucii—a spirit residing in the Ring of the Lucii) is the first king of Lucis in Final Fantasy XV Universe who lived 2,000 years ago. He appears in Final Fantasy XV (extended appearance in the Royal Edition) and Kingsglaive: Final Fantasy XV. The Mystic is the most prominent Lucii, acting as the spokesperson for the collective of kings in the movie.

Profile
His human visage unknown, as a Lucii the Mystic has holes on his helmet and two wings on top of it. The back of his suit of armor has something similar to a bony tail, like the Crownsguard does. The winged/horned helmet and the "tail" could allude to Bahamut, the Astral god who chose the Lucis Caelum line for a greater purpose in the world, and whose plan to eradicate Starscourge from the world the royal line follow. Every successive Lucii has had a similar appearance, yet unique to their personality, but the dark suits of armor they wear resemble Bahamut who presents himself similarly.

In the artwork of the Cosmogony, the Founder King is portrayed as a black-haired man wearing a golden crown and a gray cloak.

Somnus's weapon is an elaborate longsword that would become known as the Blade of the Mystic. Somnus can wield "weapons of light," conjuring them from thin air at will. He can warp and use magic, traits shared by his brother and descendants. He is said to have passed the Astrals' Revelations, and thus could likely summon them to his side.

Story
Somnus is the younger brother of Ardyn Lucis Caelum. Ardyn was to become the first king of Lucis, but after falling from the Crystal's favor and becoming the Immortal Accursed, Somnus ascended the throne instead and would become known as the Founder King. According to Ardyn, Somnus was jealous of him and thus came to demonize him, but the veracity of Ardyn's account is unclear.

Starscourge is a malady that infects Eos and its inhabitants, animals and humans alike, turning them into daemons. After Ardyn absorbed the malady into himself and became immortal, he was rejected by the Crystal and fell from grace. As the world plunged into the darkness Starscourge brings forth, the civilization fell into ruin as even the Astrals turned against each other in the Great War of Old. Somnus was served by Gilgamesh as his sworn shield, and Shield of the King would become the title given to the Lucian monarch's main bodyguard in honor of him. Somnus rose to protect the first Oracle, a maiden whom Bahamut granted powers to fight the Starscourge and commune with the Astrals.

Together Somnus and the Oracle set out on a journey where she helped him forge covenants with the Astrals, who would test his prowess with trials that laid waste to entire cities. With the Astrals by his side, Somnus is said to have collected a collection of glaives of light and eradicated Starscourge from Eos with them—likely by fighting Ardyn and likely killing him. Though Starscourge subsided, Ardyn by now was sustained by the Starscourge itself, and as long as his immortal corrupted soul would persist, the world would not be safe.

Somnus was granted the Crystal of Eos, the very soul of the star, and the Ring of the Lucii, a magical ring that allows the wielder to commune with the Crystal and collects his bloodline's souls, in time bonding them to the current living monarch. One day in in the distant future, the Lucis Caelums will have accumulated enough power to cleanse Eos of Starscourge: the final king would be the fabled True King, chosen by the Crystal. Until that day arrives, Somnus and his descendants are sworn to protect the Stone and the Ring.

The civilization of Solheim in ruin, Somnus could establish a new kingdom in its place: the kingdom of Lucis. The Cosmogony books that chronicle the legends of Eos posit that before falling into eternal slumber, having been exhausted by the war, the Six gave mankind the Ring of the Lucii. The ring is said to have passed from person to person, but none were fit to inherit its power. It is said to have found its way into the hands of a person deemed worthy, whom people thus saw fit to rule, who became the first king of Lucis. However, this account may not be entirely accurate, as the Cosmogony never mentions that Somnus's brother was to originally succeed the throne, and in fact, never mentions Ardyn at all.

Somnus was likely the first to entomb his longsword to preserve it for the future generations of his lineage, an array of holy weapons that would become known as the royal arms. Upon his death, his spirit was sealed in a colossal statue likeness of him, in present day known as part of the Old Wall. The statues would animate at the behest of the currently reigning monarch to protect the Crown City of Insomnia where the sacred Crystal the Lucis Caelum lineage is sworn to protect is kept. Somnus is the start of the Lucian royal line, and after death and passing onto his legacy to his descendants, his soul entered the Ring of the Lucii. For the next 2,000 years his spirit resides in the ring, bonded to the souls of his descendants to empower them through Armiger, now remembered as the Mystic and the Founder King.

The Mystic talks to Nyx Ulric when the Glaive puts on the Ring of the Lucii as depicted in Kingsglaive: Final Fantasy XV. As Nyx is not of royal lineage, he must pay the blood price for gaining the ring's power. After support from The Father, the latest Lucii to join the ring, the Mystic and the others accept Nyx's bargain and he gains the power to animate the Old Wall. The Mystic's statue comes alive and fights the Niflheim forces decimating the Crown City even if the Crystal has already been stolen, and is the last king standing at the battle's end. The statue's greatsword (a colossal version of the Blade of the Mystic) breaks in the final stretch of the battle against Diamond Weapon, so he tosses it aside and punches the Weapon instead. Nyx digs his kukri into the Mystic's suit of armor to follow the king warping away.

In Final Fantasy XV, Noctis Lucis Caelum, the 114th heir to the throne of Lucis, comes upon the Tomb of the Mystic at the Disc of Cauthess. His soul bonded to the first of his royal lineage, he inherits the Blade of the Mystic, which becomes his favored royal arm until he obtains the Sword of the Father. The Mystic's spirit is released from his tomb by Lunafreya Nox Fleuret, the present day Oracle, to empower Noctis for the Trial of Leviathan. Noctis eventually recovers the Ring of the Lucii and finds the Crystal the empire has stolen away, but is pulled inside into an otherworldly realm where the Astral god Bahamut tells him of his role, as the culmination of the kings of Lucis, as the True King whose blood price can save the world from the Starscourge.

Noctis spends ten years sleeping inside the Crystal to absorb its power before returning to the daemon-infested Insomnia to bring light back to the world. In the Royal Edition content, the Mystic is enslaved by the Starscourge and used by Ardyn to test Noctis. Following his defeat, Somnus asks Noctis to free Ardyn from his state as the Accursed. During the final duel between Ardyn and Noctis, the Mystic is among the Lucii who materialize beyond the Citadel.

After killing Ardyn's physical body and sending his immortal soul into the afterlife, Noctis sits on the throne of Lucis and summons the Lucii who plunge their royal arms into him. Noctis and the Lucii emerge to face Ardyn's corrupted spirit, and the royal arms eject from the former's body. The Mystic once again takes up arms with the Blade of the Mystic as the Lucii combine their powers in the true form of Armiger to destroy the Starscourge inside his brother once and for all. The sun rises, daemons disappear from the world, and the Ring of the Lucii shatters, finally releasing the Lucii from their servitude to the Astrals and the Crystal.

Blade of the Mystic
Blade of the Mystic is the fourth royal arm obtained during the main quest, found at the Disc of Cauthess in Chapter 4. It is a longsword that boosts the wielder's Strength through a unique self-buff. Its basic combo has Noctis perform myriad slashes and stabs at medium speed and area of effect. When the player stops attacking, Noctis will hold the sword in front of him and gain +100 Strength for 10 seconds. This buff can be activated outside of combat. Blade of the Mystic's aerial attack is a single slash, and its phase-counter is an uppercut slash.

Warp-striking with the Blade of the Mystic has Noctis shoot Armiger projections before delivering the strike, a strong direct downwards slash. Each projection does the same damage as the blade would in a melee hit adjusted for warp-strike distance bonus. Due to the focused nature of the physical strike, it is good for breaking normally hard-to-reach appendages.

Noctis displays a preference for this weapon after obtaining it, as he uses it the most whenever he executes an attack that requires a royal arm. The blade is well balanced and a recommended addition to any arsenal.

Sigils
In Final Fantasy XV Royal Edition, the player gains access to Armiger Unleashed by interacting with the statue of the Founder King in northern Leide, near the ruins of Keycatrich. After having collected all 13 royal arms, interacting with the statue yields The Founder King's Sigil.

In Final Fantasy XV: Comrades, the Mystic's Sigil: Graviton allows the bearer to create a powerful gravitational force that draws enemies close, while limiting their abilities.

Boss
Somnus is fought in Final Fantasy XV Royal Edition as a boss in the final dungeon.

Etymology
Somnus appears named after "Somnus", the main theme of Final Fantasy XV. It has been the main theme of the game ever since it was announced as Final Fantasy Versus XIII, and the game still has lingering sleep motifs with the main character Noctis. Lucis's Crown City being called Insomnia could thus be in relation to its founder king. However, the god of sleep Somnus/Hypnos's brother being Thanatos ("Death"), could also be an inspiration for his naming, considering Somnus in Final Fantasy XV is Ardyn's brother.

In Japanese his Lucii is known "the Yaksha King". is a class of nature-spirits, usually benevolent, who are caretakers of the natural treasures hidden in the earth and tree roots in Hindu, Jain and Buddhist texts. In the English localization, his Lucii is the Mystic. Mystic is a person who attains insight into mysteries transcending ordinary human knowledge by communication with the divine.

Trivia

 * The reason the Tomb of the Mystic is at the Disc of Cauthess at the base of the Meteor is never elaborated upon. Most kings are entombed in hidden royal tombs scattered across the realm. Many tombs have been lost across the ages, and the Mystic's should be the oldest. All royal tombs have the same design of a depiction of a king grasping the royal arm that belongs to that particular tomb; if the Tomb of the Mystic is the oldest, the others may have been made in its likeness.
 * In the English localization of Final Fantasy XV: Episode Ignis, when Ardyn mentions his true name Ignis assumes he was the Founder King. Ardyn corrects him.