Devilman: Youchou Sirene-hen

Demons possess power far beyond human comprehension, making them nearly unstoppable in battle. The only known way to defeat them is through another demon. Based on this principle, Ryo Asuka devises a plan to have his kind-hearted friend Akira Fudo possessed by a demon, believing that someone with a pure heart may be able to control the entity and gain comparable strength. After an incident at a club, the plan succeeds, and Akira is possessed by the demon Amon. Now, Akira becomes the last hope for humanity in its ongoing struggle against the secret demon threat that has existed since ancient times.