Yshiggol

Yshiggol was a mind flayer elder brain living in the Dread Vault in the Whispering Caverns.

In the past, Yshiggol was responsible for the corruption of the dwarven Clan Duergar into the subrace of duergar. When the duergar rose against their masters, Yshiggol was thought by all to have been killed by the rebellious slaves. In truth, the brain was trapped in a magic prison by duergar magic powered by devils.

Circa 1480 DR, the mind flayers discovered the truth of the situation and worked to free their master. In the end, Yshiggol was freed from the prison by the mind flayers, but soon after was killed by some adventurers.