Somnus Lucis Caelum

Somnus Lucis Caelum, more commonly known as the Founder King (as the historical person) or 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 form 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 Caelums for a greater purpose in the world, and whose plan to eradicate Starscourge from the world the kings follow. Every successive Lucii has had a similar appearance, yet unique to their personality.

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 the Immortal Accursed, Somnus ascended the throne instead and would become known as the Founder King. Somnus was served by Gilgamesh as his sworn shield, and Shield of the King from here on becomes a title given to the Lucian monarch's main bodyguard.

Somnus rose to protect the Oracle and is known to have wielded a longsword. He was likely the first to entomb his weapon 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, which 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, a royal heirloom that lets the members of the royal line communicate with the Crystal. The ring was gifted to the Lucis Caelum lineage by the Astrals to one day birth the Chosen King, and is the marker of the true ruler of the realm.

Somnus's Lucii is known as the Mystic, and for he next 2,000 years his spirit resides in the ring, bonded to the souls of his descendants to empower them through Armiger.

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 lost, 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. 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 Starscourge.

Noctis spends ten years sleeping inside the Crystal to absorb its power before returning to the daemon-infested Insomnia to return light to the world. After dispatching the Immortal Accursed's soul into the limbo between life and death, Noctis sits on the throne of Lucis and summons the Lucii who plunge their royal arms into him. Noctis and the Lucii emerge in the limbo realm, the royal arms eject from his body, and 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 his brother Ardyn once and for all. Ardyn's soul eradicated, Starscourge and 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.

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 be in relation to its founder king.