Mortem Cultus
The Mortem Cultus is a death cult devoted to the idea of mortis inevitabilis, or "Inevitable Death". The cult defies the immortality and authority of gods, incarnates, and undead and seeks to dethrone them and impose final death upon them all. Their founder was himself a lich who realized how empty immortal existence was and saw the corruption that festers when immortals rule over mortals. "The future belongs to the children" was once a common saying among the cult. The founder once managed to gather together a group of incarnate demigods to betray their creators and attack the gods. The cult claims they successfully slayed several gods and even dethroned an entire pantheon before the heroes themselves were slain by other gods.
The cult is outlawed in nearly every civilization throughout the known world. People fear and hate the cult that seeks to destroy their gods and plunge the world into chaos. Afterall, the gods bestow mortals with the very blessings people rely on to survive. Though the cult is constantly hunted, it never seems to fully die, always passing from one generation to the next...
Orcs, goblins, lizardfolk, and several other races that are common in the wilderness seem more amenable to the cult, though often sects of the cult spin the hatred for the gods to manifest as "all the gods...except ours".