Illithid

The fearsome illithid, often referred to as "mind flayers" plot from the depths of the Undervale, seeking to make all other lifeforms thralls to their will. Slim humanoids with a head that looks something akin to an octopus, these mind-controlling aberrations are alien, self-important, and megalomaniacal. It is fortunate for the surface dwellers of Valine that the creatures focus their current attention on dominating the Undervale, putting them at direct odds with other denizens of the endless dark.