Lord Karsus was the Netherese creator and caster of the Karsus's avatar spell that triggered the Fall of Netheril.

Lord Karsus was the Netherese creator and caster of the Karsus's avatar spell that triggered the Fall of Netheril.

 
“
Ah, yes. Karsus. Karsus was perhaps the most powerful wizard who ever lived. The child-who-would-become-a-god, the elves called him. And he tried. With a spell of his own devising, he endeavored to usurp in one fell swoop the power of the goddess of magic... Imagine what it must have felt like. To be a god. To know yourself to be untouchable. To be mistaken.
”
— Gale of Waterdeep

 
https://forgottenrealms.fandom.com/wiki/Karsus