Amon is the demon that possessed Akira Fudo. To his suprise upon entering Akira's body he himself became possessed by Akira's kind will. Thuscreating the first Devil Man
Amon: The Apocalypse of Devilman
Anime OVA of Go Nagai's Devilman. The story takes place towards the end of the original manga where Akira faces Amon before fighting Ryo/Satan.