Naenia

Naenia is the embodiment of a curse created by Vanitas of the Blue Moon. Set forth from the Book of Vanitas, Naenia is a mysterious entity which can invade the subconscious of vampires, distort their true names and claim possession of a vampire's subconscious.

[Spoiler] She is the malnomen of the first Vampire of the Crimson Moon, Queen Faustina. [/Spoiler]

(Source: Pandora Hearts Wikia)