Devilman/Awesome

Devilman's first battle. Frightened and maddened out of his wits due to being an human in an underground chamber filled with menacing, nightmarish monsters, Akira dashes off. Right then Amon tries to possess him... and Akira manages to defeat him -one of the most powerful demons ever- in his mind and steal his power. Straight after he transforms into Devilman and starts brutally slaying the several dozens of demons filling the room.
In the OVA "Amon: Apocalypse of Devilman", Akira loses his human heart due to a tragic loss, and thus is fully taken over by Amon. In the climax, Amon attempts to destroy Akira in a battle within their conjoined soul. Akira proceeds to show just how badass a true Devilman is.