At the beginning of the 20th century, H.P. Lovecraft wrote a series of stories about darkly powerful elder gods and eldritch horrors. These “Great Old Ones” were "ancient, powerful deities from space who once ruled the Earth and who have since fallen into a deathlike sleep". The most well known among these mythical figures was Cthulu, and even if you’ve never read a single Lovecraft story I’d be shocked if you didn’t think of giant octopus monsters and grimy insanity when someone mentions Cthulu or Lovecraftian horror in general.
There’s something uniquely appealing about a pantheon of powerful, uncontrollable, sleeping gods who if you look upon their true form, will drive you to insanity, and so even after Lovecraft’s death the Mythos has carried on, appearing in all sorts of media.
[Spoilers for Cabin in the Woods, Homestuck, Haiyore! Nyarko-san below the jump]