Eidolon was the elder brother of D'Sparil and Korax, and together the three were known and feared as the Serpent Riders, each of whom was responsible for terrorizing a different world. Eidolon plagued a world called Thyrion, subjugating its people with the aid of his generals, the Four Horsemen of the Apocalypse. Of his brethren, Eidolon was the strongest, and came closest to achieving his goals, as at the height of his power he had brought all four continents of Thyrion (Blackmarsh, Mazaera, Thysis, and Septimus) under his sway. Only the unexpected cooperation of four heroes was able to break his iron grip on Thyrion, as one by one each of the Horsemen he had entrusted to govern the continents was defeated, eventually opening the way for an assault on his sanctuary.
Though Eidolon was defeated and killed, his legacy lives on in his more devoted servants. The Demoness, a magical creature Eidolon created not long before his death, devotes her life to reviving her master and meting out revenge upon those who killed him. With the failure of the mage Praevus to absorb the soul of Eidolon and his brothers into his own, it is unclear whether Eidolon's soul has met its final end, or whether he may yet find a new vessel for his evil.
Eidolon's physical characteristics are more or less consistent with traditional depictions of devils. His skin is a brightly-colored red, he has two curved horns on either side of his head (as well as smaller ones on his back, tail, lower legs, and the crown of his head), and he walks via two hooved goatlike legs. His sinister visage is complemented by a reptilian (or possibly draconic) face with a short goatee and two fiercely burning amber eyes. At his normal stature, he is quite an intimidating presence, towering over even a tall human, though with the aid of his Chaos Sphere, he can grow in size many times over, becoming an indestructible skyscraper capable of crushing all would-be heroes underfoot.