E.N.D.

E.N.D. is an Etherious and 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 a battle 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.

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 told that there is too much Magic power present in the world for E.N.D. to resuscitate.

Abilities
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, and the fact that Igneel, the Flame Dragon King, was unable to kill it in their battle.