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]

Darkness is moving in, and young exorcist Allen Walker is humanity’s greatest hope against the wicked forces conspiring to bring civilization to its knees. Akuma – cruel spirits born of tragedy and lost souls – lurk in every shadow, willing and eager to do the bidding of their leader, the dread Millennium Earl. With an eye cursed to see evil in its truest form and blessed with an arm to slay soul-devouring demons, Allen stands ready to confront the gathering evil. Should he fail, Innocence will be lost forever. The war to decide the fate of mankind has begun – and the carnage will be endless.

[Source: Funimation]