The High Master Illithid is the main antagonist for most of the module Thoughts of Darkness.
The High Master Illithid has long wished to usurp his master, the Lord Elder Brain. His plan is to become an Illithid Vampire with the powers of both species. He made a pact to overthrow Strahd Von Zarovich in exchange for vampirism with Strahd's grandniece Lyssa. However, his alien physiology was incompatible with vampirism and the only way to truly create one was to turn a vampirized individual into a Mind Flayer and such creations were insane and had to be destroyed immediately. When a member of The Brotherhood of the Contemplative Order came, Lyssa learned of a device called the Apparatus that could transport the High Master Illithid's brain into a Vampire Illithid tadpole. However two members of the order, Anitella and Lord Drassak came and stole and hid the Rod that powered the device.
In the Adventure Edit
If the characters play the adventure right, The High Master Illithid will transfer his brain but be destroyed by the Lord Elder Brain.