Karsus

Karsus was an ancient and powerful wizard of Netheril who developed a 12th-level spell, Karsus' avatar, that could steal the power of a god for the caster. However, his desire for power overwhelmed his common sense, and he decided to use the spell to steal the power from the goddess of magic, Mystryl. However, once he absorbed her power and knowledge, he realized the mistake he made as magic went mad across the whole of Faerun. To save the world, Mystryl sacrificed herself, and Karsus, after seeing the destruction of the floating cities of Netheril that were his home, died as well. His power-engorged body transformed into a red-hued mountain, and his spirit remained trapped with it, to repent his folly forevermore.

For a brief time, survivors of Netheril worshipped Karsus as the Momentary God, hoping that he would rise again and bring back their nation's glory, but when all their prayers went consistently unanswered, the cult died out. Since then, Karsus has languished, racked by guilt, alone.