In a time unknown, only a prophecy keeps hope alive in people's hearts. 'When darkness veils the world, the King of Light shall come.'
True King, also known as the King of Light, the King of Kings, or the Chosen King, is the king who wields all of the Crystal's power in Final Fantasy XV. The True King has the power to banish darkness from the world.
The True King is homage to the concept of the Warriors of Light, one of the core concepts of the Final Fantasy series of heroes chosen by the crystals—be it by a prophecy, a will, or by fate—to embark on the journey to fight evil or restore balance to the world.
To crown the King of Light is the calling of the Crystal. Only the True King anointed by the Crystal can purge our star from its scourge.
Eos is world whose soul is inside the Crystal protected by the line of Lucian kings. The kings wield its power via birthright through the bonding of souls, and via the Ring of the Lucii that saps the king's own life force in exchange. When the young Noctis Lucis Caelum spent time with his friend, Lunafreya Nox Fleuret, she told him that he will be the True King chosen by the Crystal to banish darkness from the world.
Eos is plagued by Starscourge, an encroaching darkness that has daylight vanish from the world and turns the life on Eos into daemons, mindless monsters. The True King's calling is to purge the darkness at the cost of his own life.
Little by little the power of the Lucian kings is concentrated on the Ring of the Lucii, each wielder able to call upon the power of his forebears. When all of the Crystal's power is controlled by the ring, the wielder is the True King, whose power is said to be greater than the power of gods themselves.
It is implied that there was a Chosen King 2000 years ago, when Starscourge was advancing rapidly and many were dying. The king who was given the Crystal to protect, along with the Ring of the Lucii, is said to have fought the darkness alongside the astrals, as spoken of in Cosmogony. Final Fantasy XV Scenario Ultimania posits that the plague subsided by the king's hand. The plague was brought back in a large scale however after Niflheim Empire began experimenting with daemons.
On M.E. 741, King Regis was told by the old kings that his son Noctis is the True King. The Final Fantasy XV Scenario Ultimania posits that than being able to use the Crystal's true power, kings of Lucis have the responsibility of protecting it, and that the Crystal picks the "Chosen King" when disaster comes to the world. The Crystal chose Noctis.
On M.E. 756, after the Crystal is stolen from Lucis by the Niflheim Empire and King Regis is killed, Noctis follows his calling as guided by Lunafreya, an Oracle able to commune with the gods. Noctis sets out to collect the legendary weapons of his ancestors and to forge covenants with the gods of Eos, the astrals.
After Lunafreya dies at the hands of Ardyn Izunia, Noctis's immortal ancestor who was rejected by the Crystal for the darkness in him, the Starscourge begins to engulf Eos even more rapidly and daemons grow fierce and more numerous. Noctis is desperate to find the Crystal for its power is said to be able to banish the daemons from the world, but he is not aware of his role or the meaning of being the True King. When Noctis finally finds the Crystal with Ardyn's help, to his surprise he is absorbed inside it. He learns the Crystal houses the soul of Eos, and that he is the True King who can save the world at the cost of his own life.
Noctis sleeps inside the Crystal for ten years, and after he returns he discovers the sun never rose again after he was gone and that daemons have taken over the world. He reunites with his friends and together they make their way to the ruins of Insomnia to where Ardyn has taken the Crystal. Ardyn's soul is unable to rest in the afterlife due to being corrupted by daemonic influence, and thus he has become immortal and the embodiment of darkness in the world. He ushers Ifrit upon Noctis's party, the astral known as The Betrayer who is the source of the Starscourge plague and the daemons, but they prevail with the help of the other astrals.
As they approach the throne room at the Citadel, Noctis and his friends have a chance to gaze upon the paintings of the prophecy. They depict the True King (Noctis) and his three friends, one of them with bandaged eyes (Prompto, Gladiolus and Ignis), with the angel of the Oracle above them (Lunafreya). On the background stand the Lucian kings of past, and on the sides the astrals excluding Ifrit. The bottom of the painting has the daemons, shielding their eyes and being repelled by the True King's light.
Noctis finds Ardyn sitting on the throne of Lucis under the Crystal, and the two duel both wielding the power of kings. As Ardyn dies, he fades away and asks Noctis to meet him "on the other side". Noctis, not wanting his friends to witness his death, orders them to stand guard outside while he ascends the throne below the Crystal. He fulfills the prophecy by calling upon the souls of the past kings via the Ring of the Lucii, who attack him until his soul is also taken by the ring. Finding Ardyn's spirit, Noctis and the combined souls of the past kings erase Ardyn. The Ring of the Lucii breaks, and the sun rises.
As implied in the Omen short film, King Regis knows Noctis will be the True King; when he speaks with the Crystal it says it is looking for the last life—King Regis's. The Final Fantasy XV Scenario Ultimania confirms that Regis learned of Noctis's destiny fifteen years ago.