The Dry One
The Dry One was an illithid who achieve lichdom. He also had very potent psychic powers, but was a master negotiator and planner. The Dry One’s biggest advantage was that over the years as a lich he discovered many items of note, in particular the Annullus.
The Dry One’s true name is unknown. He lives in a secluded section of the underdark away from other illithids. He was banished from the society based on his decision to become undead, a decision made by the Elderbrain. The Dry One wondered the Underdark honing his talents and collecting information about the world. Eventually he found a nest of illithid who he took control of. His ultimate goal was to become the leader of his former home but needed to rid it from the Elderbrain. While The Dry One did have impressive psychic powers he did not possess the personal or political powers to defeat an Elderbrain. After decades of searching for a method to get rid of his problem he discovered the Annullus. The Annullus is a psionic artifact that was created to bypass psionic powers and had the ability to annihilate all psionic creatures in a radius, such as the Elderbrain.
His compound was being attacked by an independent military force when he realized that the PCs joined the fray. The PCs hated illithid but were given an option to speak with him if they helped him get rid of the duergar force. The duergar was a specialized unit in defeating other psionic creatures, illithid especially, but the PCs were not psionic and thus the duergar would have no advantage. Grazit was the only undead in the group so he wished to speak with him, and after they agreed to help one another. The PCs killed the duergar and were given the Annullus. They agreed to use the Annullus to kill the Elderbrain, which meant they needed a major assault on the illithid city so they could get close enough to set off the psionic bomb.