The Millennium Earl is intent on the destroying the world using the power of the Akuma, biomechanical weapons made from a mechanized skeletal core housed within a human body and powered by the souls of the dead.

Since his youth, Allen Walker has been cursed with an anti-Akuma weapon as his left arm, and an eye that sees the former soul of the Akuma. He is also one of the few who contain "Innocence", a substance that allows him to kill Akuma and fight the Earl. With these powers, he joins the Black Order as an Exorcist and sets out with other Exorcists to stop the Earl's attempts to annihilate the human race.

[Source: AniDB]