Eligos was a demon, a duke, and a knight of the Order of the Fly. He used to command sixty of Hell's legions, but after his defeat at Hellboy's hands on the bridge of Morgan Le Fey's castle he was stripped of his rank and power and cast down into the pit.[1]
Injured by Hellboy, the duke was discovered by Rasputin, who with the help of the Ogdru Jahad, managed to revive the demon, and told him where to find Hellboy, when he would eventually arrive in Hell.
When Hellboy arrives in The Abyss, on the outer edges of Hell, Eligos attacks him again, this time out for revenge. Eligos lays a beating on Hellboy before he is rescued by Edward Grey. Eligos pursued the two to Grey's haven in Lethe. Edward Grey gets Hellboy clear and later tells Hellboy that he trapped Eligos there; hopefully forever.
At some point after this he would be given power by Pluto, the original ruler of Hell, that would and became himself as a servant for Pluto to reclaim Hell and possibly even other realms. When Edward Grey returned from Earth after Hellboy caused Ragna Rok, he was terrified to see what had happened to the demon and begged him to denounce his service to Pluto, claiming that his return would destroy both Hell and Eligos. Eligos himself saw Pluto's return as Hell's salvation, and it is revealed that the late Beelzebub had managed to awaken the trapped king.
Eligos sees Grey as a threat to himself and his new master and tries to kill him with his powers, though becomes interested in the story of the man. Sir Edward Grey reveals that his humanity is long gone, and he is no longer the human he once was, not being able to die after his last encounter with Rasputin and being filled with endless power. Saying this, grey attacks Eligos, and destroys him, becoming officially the personification of the river Acheron.[2]