The infamous brainsuckers or mind flayers of the Underdark can be recognised by the tentacles in their faces. Don't let the vaguely humanoid shape fool you, these monstrosities will mesmerize you and eat up your intelligence before you can say "psionics"!
Like many other denizens of the gloomiest caverns known to man, illithids are no strangers to slave trade. They can force almost any creature to be their thrall or cattle using their psionics and wicked inventions.