(or, what I use when I need some Lovecraft in my game)
The Isilithil are an ancient race of eyeless subterraneans who despise loud noise. The caverns under their control are vast and quiet, often haunted by other silent creatures. They are roughly a head shorter than most humans, with a mass of tentacles on their face and no other visible features. Their skin is cold and clammy.
While their technology has become primitive over the centuries, the remains of their dull-grey rock cities remain, enormous and monolithic, on the shores of deep underwater lakes. They often commit bloody sacrifices of captured humanoids, and legends abound of entire towns claimed in the middle of the night. Those who have observed them in their lairs and lived (a rare feat) report that the sacrifices are accompanied by horrific dances around silent fires.
Their principal ability is to cast a Zone of Silence spell (15'-30' radius) at will. They attack in other ways as Grimlocks, but tend to prefer to follow their prey at a distance for some time. They will investigate any loud noises in their chosen caverns, and occasionally mix strange poisons for their blades. No-one knows how (or what) they eat.