This Mind Flayer was encountered in the Xanathar sewer hideout, silently watching over Grum'shar, the half-orc wizard, torturing and interrogating Floon.
 
When the players interrupted the interrogation, the Mind Flayer calmly stood up, set down one of his pet Intellect Devourers, and used a mysterious stone sphere on a dias to open up a temporary portal to somewhere unknown.
 
Before the Mind Flayer got away, he exercised his mind control powers over the strongest party member, forcing them to attack the other heroes.
 
He hasn't been seen since that harrowing night.