Mastermind

The Mastermind is the leader of the Thieves Guild. The current Mastermind is Shissa. 
Due to a wish made at the Haunted Well, Gizmo genuinely believes themself to be the Mastermind of the Thieves Guild.

Appointment

A player can become the new Mastermind if they climb up through the ranks to Villain, are at least Level 17, and complete a special quest, probably stealing something from a rich person's very secure house.