The setting for many of GURPS' fantasy sourcebooks, Yrth is a magical world whose primary continent is the Europe-sized Ytarria.
Once occupied primarily by elves and orcs, a dark elf splinter group created a magical vortex known as a "Banestorm" designed to remove the orcs permanently from Yrth. The spell backfired badly, and resulted in races from other dimensions and planets suddenly appearing in Yrth, including medieval humans (bringing Christianity, Judaism, and Islam with them), reptile men, insect men, and many more. Humans quickly adapted to the magic of Yrth and their empires and new religions quickly drove back both orcs and elves alike. Ytarria is now drawn into hard borders not unlike medieval Europe of the Crusades era.