The Serpent Riders are the main antagonists of the first three Heretic games. They dominated the seven kings of Parthoris into mindless puppets, causing them to lead their subjects into doing the Serpent Riders' bidding. Only the Sidhe elves had no allegiance to either of the seven kings, so they weren't affected by the Riders' spells.
The Serpent Riders are (from what is both youngest and weakest to oldest and strongest, and in which game they appeared in):
Only D'Sparil remained in Parthoris, while Korax dominated the world of Cronos, and Eidolon dominated Thyrion, another world.