The Great Whisperer (illuun)

An aboleth living in a strange obsidian lair deep beneath the pyramid of Nesme, Illuun had been causing chaos in the Silver Marches for at least 15 years. Recruiting Duergar, mind flayers and intellect devourers to serve him, he sent his minions across the region to sow discord. One of his earliest minions may have been the unknown wizard who constructed the pyramid and accidentally came within the reach of the aboleth. The necromancer Aldred Blackwood also came under his control and came to be the host of an intellect devourer.
After entering the pyramid and defeating the necromancer, the Forsaken Company came under his influence and began having horrific nightmares that slowly drove them mad. After dealing with several of his minions in Nesme and Mithral Hall, they ventured into his lair in an attempt to defeat him and end the nightmares.
After venturing through his lair and battling eldritch horrors, they found Illuun in his chamber and slew him after a long and difficult battle. The flumphs that lived around him revealed that his obelisk had crashed to abeir/toril thousands of years ago, and the city of Nesme had been built atop it. Illuun slept until the mage Euclidius began building his pyramid, at which point the mage delved too deep and awakened him. Illuun enslaved him, creating his first minion on his crusade of domination that would built up over 15 years until his death at the hands of the Forsaken Company.