Demon Knight (悪魔騎士, roughly Akuma Kishi) is a full-color prequel manga to Devilman by Go Nagai. It was published in three parts annually from March, 2007 to 2009 in MANDALA, a yearly special edition of Kodansha's Morning magazine. It tells the origins of Amon, Sirene, Satan, and the entire Demon race in the ancient past. In 2017, the manga was collected in the first volume of Gogasha, a collection of Nagai's short stories and one-shots.
In the ancient past of Earth, the being called God established his domain. Watching Earth in his place is his prince Lucifer who is helped by his Demon Knights, Amon and Sirene. The land is inhabited by clans of various people including the therianthropes and the People of Paradise as God had determined.
Soon the demons appear from a mountain range and the Demon Knights and Prince set out to combat them. Fighting against the demons, the group encounters a young girl from the people of paradise named Yuria and the werewolf Urga. Lucifer explains that the demons themselves were failed genetic experiments from long ago that were sealed within the mountain. The most terrifying ability the demons possessed was the ability to merge. This gave them great power but also made them into horrid monsters.
In battle against the demons, Sirene had fused with her bird mount after it was slain. Lucifer had fused with Yuria and his slain pegasus becoming Satan. Satan had found Amon and forcefully fused him with Urga and his dragon companion to join them as this was practically an easier way of evolution.
In the present day, Akira Fudo explains to his friend Ryo Asuka about a dream he had, revealing it was actually a memory of Amon during these events. Ryo also had the dream in spite of not fusing with a demon. Ryo then says that Akira's battle with Sirene was won purely with luck after she fused with Kaim, suggesting that Akira do the same. Akira however, does not want to become more of a monster and leaves with Ryo left to wonder what is happening.