Devilman

Based on Gô Nagai's manga, about mild-mannered teenager Akira Fudô, who merges with a demon to fight other demons.
A scientific exploration in search of a new energy source unleashes a group of powerful demons who begin possessing human bodies and destroying civilization. Close friends Akira and Ryo wind up on different sides of this epic struggle. Akira’s inner strength allows him to maintain his identity and fight for good, but the weaker Ryo is completely overtaken by Satan himself.