The Soul Proprietor
This mindflayer is larger than most and runs all aspects of the Fatespinner's Arena. You feel slightly less lucid in its presence.
Mind Flayer Mastermind (MM 188)
The Soul Proprietor relishes in the art of combat. Bored with having absolute control over the minds of its thralls, the Proprietor began gathering creatures, criminals, and would-be adventurers from around the realm. It pits them against each other in vicious death-matches in its arena. Many adventurers are saved from near-death experiences, prison, and exile by the Proprietor. By surviving and impressing the Proprietor, adventurers are set free with all prior criminal records revoked. Many who survive refer to the Mind Flayer as The Fatespinner for altering the course of their lives.
It offers the spoils of past fallen heroes as rewards for excelling in the arena.
The Soul Proprietor has also been known to personally challenge exceptional adventuring parties to battle, rewarding its entire treasure trove to those who defeat it. Though the challenge grows more irresistible over time, none live to claim the bounty.