Parthoris was invaded by the Serpent Riders, who corrupted the other races of Parthoris into their service and turned them against the Sidhe. When attacked, the Sidhe elders blew out their mystic candles, destroying the armies of the Seven Nations. This caused D'Sparil, the lone Serpent Rider left behind to rule Parthoris until its doom, to retaliate with genocidal warfare.
After Corvus's defeat of D'Sparil and his wanderings in the outer worlds, Parthoris was again burdened with strife. In Heretic II, a Seraph named Morcalavin unintentionally fell under the lingering influence of D'Sparil by using a False Tome of Power in an attempt to ascend to immortality. He unleashed a devastating magical plague which drove most of the world's inhabitants mad with bloodlust.
Much more of Parthoris is revealed in Heretic II than could be observed in the original game, as the original took place largely within the Silverspring area, as well as on the bed of the sea (some of the episodes do not take place on Parthoris at all, but in other worlds long conquered by the Serpent Riders). Its terrain is seen to include vast oceans, swamps, deep canyons, dark caves, high mountain ranges, and vast unexplored wastelands.
Amongst the diverse races living on Parthoris, aside from the Sidhe, are the amphibian Ssithra, the insectile T'chekrik, and the dwarf-like Ogles.