Discworld/Eric/WMG

It was the events of Eric (and the end of Sourcery) that resulted in Rincewind's Immortality and temperment.
Rincewind was cast into and STUCK in the Dungeon Dimensions and then by freak accident summoned and reversed summoned through a Summoning Ritual that gave him demonic powers. Hence, his extreme Post Traumatic Stress Disorder that he is known for is first seen in this book; and the details of the summoning combined with having been Place Beyond Time when Discworld was created changed his metaphysical state.