Seylos was a powerful demon general of a small demon army, he only appears in Amon: Apocylpse of Devilman as one of the main antagonists.
Amon: Apocylpse of Devilman
In the OVA, Amon is first mentiond by Yumi after getting back from a scouting misson saying that he was going to kill Akira and the other Devilmen, not long after Akira Fudo had found the dead decapitated bodies of Miki and Tare Makimura. Almost imedieatly after Seylos and his demons crash throgh the roof of their hideout, he demands that Akira Join him or die with the others but Akira full of rage loses control of his feelings and releases Amon the Demon Akira had origonally gotten all his powers from.
The hideout is destroyed and Amon kills Yumi, and Seylos sends an armarda of his followers to try and kill Amon, unfortantly for them Amon is to strong and kills many demons before there were only four left, he finds the Devilman Miko trapped under some rubble and moves into kill her but notices the demons flee, instead of killing Miko he flys after two of the demons and massacres them, Seylos kills the other two for fleeing from the fight just as Amon comes, the two fight across the city, Seylos cuts throgh several buildings with his Devil beams but Amon throghs a train at him knocking him out of the sky, eventually Amon snaps Seylos wrists flies up and impales him with his fist through his stomach killing the demon.