Devilman

Devilman: Young Akira fudou is suddenly possessed by a very powerful demon. Transforming his entire body between half demon and half man, but still retain his humanity. With he becomes The Devilman: fighting demons hidden in the modern world and preventing the destruction of mankind.

History
Akira's parents are killed on a trip to the arctic and he lives with Miki Makimura's family. A friend of Akira, Ryo Asuka, tells Akira that he needs to come with him and tells him about the existence demons. After being persuaded by Ryo, Akira decides that he must be possessed to save the world. After a ritual/party with music/booze/semi-nude women, Akira is possessed by Amon, one of the strongest demons, and kills the demons (whom many were disguised as party-ers). Later, Akira fights Gelmer, Jinmen and Silene (who was Amon`s GF) and the combined form of Silene/Kaim. Later on it is revealed that Ryo was the great demon god Satan. It is also revealed that Ryo has flushed feelings for Akira. Later, when the demons are attacking mankind, Ryo tells Akira to stay out of the war and goes on TV to expose Akira as a demon (with a home video of the nightclub thing...only Akira wasnt a celeb). Miki and her little brother gets killed by a mob who believed that they were demons because Akira was with them. Akira arrives too late to save them and kills the mob in a bloody massacre. Akira then gathers together an army of devilmen and fights a 20 year war against Satan. The war later ends with Akira being severed at the torso. Satan regrets his actions and for killing the one he loved (Akira).( From Go Nagai Wiki)