The Dawn Of The Future (Spoilers)
Ardyn chooses to defy Bahamut, and is tortured by a vision of Aera conjured by the Bladekeeper to inflict the maximum amount of pain. Upon being returned to his cell in Angelgard, Ardyn rejects the gods' plans and vows to become strong enough to kill both the True King and the Astrals.
His exploits in Gralea are given more detail. On the empire's "last day", he makes the magitek infantry go berserk, and unleashes a horde of daemons and a Diamond Weapon that begin tearing through the city. He broadcasts a message that includes the dying Emperor Aldercapt's final words of the glory of Solheim rising again. When Aranea confronts him, Ardyn brushes her off and soon leaves the city to its fate to await Noctis's arrival.
Ten years later, he waits in the Citadel's throne room for Noctis. To his surprise, the one who walks in is Lunafreya, resurrected to aid Bahamut in destroying Ardyn and absorbing the world's darkness. Knowing Bahamut now seeks to purge Eos of life, Lunafreya intends to thwart his plan with Ardyn's cooperation. Ardyn is uninterested in an alliance, and when Lunafreya relays Aera's wish to save him, he angrily summons Ifrit to kill her. Lunafreya absorbs the Scourge from Ifrit, and makes a covenant with him. When Noctis arrives Lunafreya has fully turned into a daemon, as her power to absorb the darkness is the same as Ardyn had used 2,000 years ago. Ardyn mocks them as Lunafreya attacks Noctis.
Bahamut commands Lunafreya to unleash the darkness to power Tera Flare, his ultimate attack that will destroy humanity and the other Astrals. As the Citadel is raised to the sky to protect the Crystal, Ardyn warps to it in the knowledge that Noctis will try to reach Lunafreya. When Noctis arrives, Ardyn relates Lunafreya's plan to force Bahamut to unleash Tera Flare, which the other Astrals would block, forcing Bahamut into slumber as his true death is impossible due to him existing within the Crystal. Ardyn continues to be defiant, attacking Noctis after asking for the Ring of the Lucii. Noctis realizes that Ardyn plans to use the power of the Kings of Yore to destroy Bahamut's spiritual self.
Ardyn is defeated, but Noctis stops short of killing him. While he hates Ardyn, he concedes that he wishes to save him and that his only salvation is death in defeating Bahamut. After being given permission from the Kings of Yore led by Somnus, Noctis gives Ardyn the ring. As Noctis fights Bahamut's physical form with the other Astrals, Ardyn ascends the throne and undergoes the ritual to travel to the Beyond, just as Noctis would have done. The final blow is delivered by Somnus's spirit. Now in the spirit world, Ardyn summons the power of Providence and destroys Bahamut there. Before his soul disintegrates, Aera speaks to him one last time, vowing to remember him. In his last moment, he asks her to always be with him, and hears her reply before fading away.