Mindflayer from Garm's flash back
Illithids (commonly known as mind flayers) are monstrous humanoid aberrations with psionic powers. They believe themselves to be the dominant species of the mortal plane and use other intelligent creatures as thralls, slaves, and chattel.
Khardaghk led an assault on a Gith monastary. He controlled armies of trolls, ogres, orcs and other monsters. He mercilessly slaughtered all the people who the TFG became close to in the dream.
Khardaghk recently resurfaced when he pitched the party against experimental creatures that ate adventurer’s gear.