E.N.D.

E.N.D. (ＥＮＤ, イーエヌディー, Ī Enu Dī) is an Etherious, the strongest Demon created by Zeref, and the Guild Master of the Dark Guild Tartarus.

History
Four hundred years ago, E.N.D., the strongest and vilest Demon born from the Books of Zeref, fought with Igneel, in which the Dragon failed to kill the Demon. At some point, the Etherious formed the Dark Guild Tartarus with E.N.D. as their Guild Master.

However, under unknown circumstances, E.N.D. was sealed back into his book with an unbreakable seal four hundred years ago.

Tartarus arc
With E.N.D. in a dormant state, the Demons of Tartarus seek to revive their Guild Master by wiping out the Magic Power in the world and gathering Curse Power, and return to their creator Zeref's side.

After the destruction of Face, Kyouka reports to Mard Geer, who is holding E.N.D.'s book, requesting that they wake their master up, only to be told that there is too much Magic Power present in the world for E.N.D. to resuscitate.

Abilities
Aside from being a Demon specialized in the use of fire, the exact power and abilities of E.N.D. are currently unknown, though the likely extent is suggested by Zeref's description of it as the strongest Demon he ever created..