The leader of Waterdeep's Thieve's Guild is a Beholder with a ton of racial HD (and sorcerer levels, for.... reasons?). He has a mutation that allows him to not be irrevocably insane and paranoid. Just insane and paranoid enough to head the largest thieve's guild in the Realms without killing all his subordinates. This is a result of a mutation that allows his emotional brain to transmit all sensory info to his logical brain without any filtration (normally, Beholder's emotional brain filters sensory input to protect the logical brain's ego) allowing an unbiased view of the world (normally, Beholder's get shown that they, individually, are the ideal).