He appears at the end of the first hub. you gain access to his lair through a red portal after defeating the golems and breaking the Crystal Wall. He is the weakest of the Four but with 2200 health points and some nasty missiles is still quite a challenge.
After the defeat of the first two Serpent Riders, D'Sparil and Korax, Eidolon the third and last of the Serpent Riders gathered together the remnants of their forces and added them to his own. He corrupted the Priests who guarded the seal that prevented the Four Horsemen of the Apocalypse from entering the world and released them to serve as his generals, giving each dominion over one of the four nations of Thyrion.