I found a new leak!! It's from a Japanese board and he claims to be a friend of CG artist working on FFXV.
Ardyn is a messenger of Chaos who was to sent to Eos so that he could prepare for the revival of Chaos. Chaos is the one who is controlling the Plague of the Stars and wants to destroy Eos the Goddess for imprisoning him. The plague of the stars is to destroy the light of Eos (goddess) which is the stars themselves, but the stars are protected by the Archeans with Eos (planet) being the core of the light of Eos.
Ardyn uses the anger and hate growing within Ravus to prepare him to be a vessel to resurrect Chaos. But before that, Ravus whose anger is only going stronger fights Noctis, but loses and snaps out of Ardyns control. Realizing what he had become, to protect Luna and redeem himself for his mistakes he agrees to help Noctis to defeat Ardyn. Ravus returns to Ardyn in an attempt to kill him, but it backfires, allowing Ardyn to regain control over Ravus making him stronger than before.
In order for Chaos to destroy the light of Eos (plague of the stars), Ardyn helps Noctis to gain the trust of every Archeans so that Noctis can bring all the Archeans to Chaos, so that he can either control/kill them in a single strike. Noctis who assumes it's his destiny pacts with every Archean, but Chaos uses this to his own advantage.
Gentiana is the messenger of Eos, who helps the Oracles to ascension, assisting in keeping the balance of the light of Eos.
Glauca is revealed to be Cor who is Drautos estranged brother. Cor after finding out that Drautos was Glauca, who killed Regis, took on his helm to infiltrate Niflheim. Giving Noctis crucial information about Niflheim activities. Glauca (as Cor) then appears to Noctis, telling him that he killed Regis and Clarus, forcing a fight between them. At first Noctis is too weak to defeat Glauca, but after through numerous confrontations, Noctis kills him. As to why Cor pretended to be Glauca is so that he could train Noctis to become stronger by posing as a stronger threat. And also that Cor felt guilty that he could not help his brother overcome his hate towards Lucis and not being able to protect his King as a result. This was a punishment to himself for his failure.
Cor tells Noctis everything before he dies, telling him that Ardyn had all the answers.
Noctis along with Luna (and Gentiana) goes to Niflheim where Ardyn and Ravus is waiting for them, but at the same time Ardyn completes the ritual resurrecting Chaos into Ravus' vessel. Chaos now in the body of Ravus, kills Luna (and Gentiana) to anger Noctis. The anger and pain of losing both his loved ones, his powers overflows him. After a chaotic battle, Noctis is defeated falling into a deep coma.
With the death of both the Oracle Luna and Gentiana, the balance of the Light of Eos becomes unstable. Darkness starts to over weigh the light.
10 years pass, Noctis reawakens by a calling from Luna, telling Noctis that his destiny is yet fulfilled. Noctis and his comrades makes a return to Insomnia where Chaos is residing. Outside the Capital square, they meet Chaos taunting them. After a short fight, Chaos retreats and Noctis makes their way into the building.
Inside, they see Iedolas sitting on the throne, dying. He said that Ardyn had promised him eternal life so that he could rule as the King of Lucis gaining it's powers and the crystal was required for him to do so, but had realized he was only being used. Having gained his dream of ruling Lucis, Iedolas dies.
But from the shadows, a shocking figure emerges. King Regis appears in front of Noctis, only it's a former shell of what he once was. Chaos had resurrected a empty shell of Regis from his corpse.
Noctis and Regis fights, where Noctis is confused by his emotions. But ultimately kills the shell of his former father, but Regis had retained some of his former self telling Noctis that he loves him and he should fight not for his father, but to save everyone, for the future.
Noctis then aims his blade at Chaos for the final fight, but Chaos drains the power of the crystal to gain tremendous power. Chaos then kills and consumes all the Archeans who comes out to stop him, completing the plague of the stars. The light of Eos had vanished and darkness befalls the planets as Eos (planet) slowly starts to crumble.
In a last ditch of effort, Noctis clashes with Chaos, but his powers overwhelms Noctis. Struggling to continue, Luna appears telling Noctis that it's not too late to save the world, giving Noctis her remaining powers. Then the spirits of the previous kings appear along with Regis, each striking Chaos with Noctis and Regis delivering the final blow. With Chaos defeated, the light of Eos returns and the balance is restored.
Ardyn watches over everything as if he enjoys it. He tells Noctis that everything was just a game for him, angering Noctis driving his blade into Ardyns chest. Smiling to the very end, Ardyn speaks, ''Finally free...''.
Years later, Insomnia is being rebuilt and Noctis is seen sitting on the throne.