Devilman was a manga first published in 1972 through Weekly Shonen Magazine, written and illustrated by mangaka Go Nagai. Released alongside the anime of the same name by Toei Animation, Nagai's manga would end up with a more mature, darker take on the story and the superhero tropes that came with it that would make it stand out from other manga of the time. Despite its run being somewhat cut short (but being just enough that a proper ending could be written), it has ended up being one of the most best-selling manga of all time and its influence can be seen and felt in all sorts of other media (including other anime and manga considered to be just as influencial, such as Studio Gainax's Neon Genesis Evangalion).

For a plot synopsis; taken from Wikipedia: Akira Fudo is a shy teenager who lives in his friend Miki Makimura's house as his parents work abroad. One day, Akira's childhood friend Ryo Asuka reveals to him that the Earth is about to be invaded by demons, monstrous beings hibernating for centuries in the ice that are about to return to the surface. According to Ryo, demons have lived on Earth before the appearance of man, and now they want to claim possession of it. The only way to defeat them is to take control of the powers of the demons themselves to fight them on equal terms. Ryo then involves his friend in a ritual called the Black Sabbath, an event where numerous demons plan to merge with humans to infiltrate society. During the Black Sabbath, Akira merges with Amon, a demon warrior both idolised and feared among his kind for his incredible strength. However, instead of Amon holding control over Akira, the latter's pure soul triumphed over that of Amon, bringing the demon to heel and creating Devilman.