Karsus's avatar was a very powerful spell which altered the very disposition of reality.
The spell was even capable of allowing its user to become a god of their choosing, replacing a current god with themselves the moment the spell was completed.
It was so complex that the wizard had to put a stone-filled gizzard of a gold dragon into a mixture of tarrasque blood and 12-headed hydra bile to just enchant one of the material components of the spell. It had a casting time of six hours.
The spell's origin was unknown, yet dates back to Netheril. Karsus's original notes regarding its creation were never found. Whether the gods were able to defend against the spell, or were aware of its casting, are unknown. Karsus spent well over a decade researching this spell. It was believed Mystra, the reincarnated form of Mystryl, snatched the spell information from the ruins of Karsus's enclave and sent it on an eternal journey to the ends of the universe.
The spell was not supposed to make the caster permanently into a deity. The spell merely allowed the caster to temporarily merge with the deity and wield their powers as long as the spell lasts. While the Netheril: Empire of Magic sourcebook mentions the limited duration during the background information about the spell, the Powers & Pantheons sourcebook wrote-up the spell where the duration was set to "limited".