Nightmare is the fifteenth, and final boss in Requiem of Agony. He is a massive demonic creature, created when the Dread Lord of Bane attempted to absorb the full powers of both Dracula's Castle and the Netherworld. He looks like a mix of Vadriel, Omega, and Mortus' most powerful forms. Bits of the castle can be seen, fused with his organic tissue. He is the most powerful character known in Castlevania fanon, due to the fact that he has the powers of Vadriel, Omega, Mortus, Dracula's Castle, and the Netherworld, followed by the Dread Lord of Bane, then Omega, then Vadriel. Giga is not counted in this list, due to the fact that he has not been seen yet.
Nightmare has the Dread Lord of Bane's attacks. He can also summon energy meteors that spawn mini-Demonic Megiddos, shoot giant energy beams from his palms, and use his long claws to attack. He can also shoot lightning, and stop time. One hit from him does around 200 - 500 damage. After defeat, he can cast Giga Demonic Megiddo, only prevented by using the true power of the Vampire Killer by pressing up, up, down, down, left, right, left, right, and the attack button. He has 14000 HP, and his theme is a remix of Proof of Blood.
Nightmare dies, but laughs, saying that he has achieved his purpose. After the ending, Omega is seen, along with Despair, Oblivion, Maldus, Alpha, Werebane, the Eye of Death, and Xeroth, many of which who were thought to be dead, in an underworld ceremonial chamber. Omega takes the golden orb which houses the true power of the Vampire Killer, and puts it in the demonic skeleton of Vadriel. The room fades as Vadriel takes form, finally experiencing full resurrection.
- During Battle: "Child, you look weary. I will lift the burden of life from your shoulders."
- During Battle: "Why do you fight to save the world? You are scum in the eyes of all you wish to save. Swing your whip. Throw your daggers. Do what you will. They will not save you from ridicule. They will not save you from pain. They will not save you from death. But I can save you from life."