It has recently come to my attention that cyanwitch
does not know who Cthulhu is.
This must be fixed.
Okay. A long time ago, there was this guy named H.P. Lovecraft. He was an author who had an exceedingly dim view of humanity, writing stories that primarily revolved around otherworldly horror and the terrifying magnitude of the universe. Despite his racial prejudices, which were a lingering reflection of the times (not that that's an excuse), his work left a huge impression on readers and the authors who followed in his footsteps. In fact, he's one of the most famous horror fiction writers -- and according to many sources, considered the greatest alongside Edgar Allen Poe.
His writing tends to be overwrought and adjective-laden with tons of run-on sentences and no full stops in sight, but something about it is weirdly readable. I wouldn't call myself a fan, but "The Call of Cthulhu" is his most famous story and one of his most creepy. It's about this gruesome creature named -- you guessed it -- Cthulhu, who wakes up and gradually causes the men around it to go insane. Through the madness induced in their dreams.
In short, it's a cosmic horror
Cthulhu itself is described as a cross between an octopus, human caricature, and a dragon, with a tentacled head and grotesque, scaly body about as large as a mountain. It's pretty hideous. I seem to remember characters in the story dying when they looked upon its visage.
Because of its influence on cosmic horror fiction, there are a bunch of related tropes
that make reference to it (and Lovecraft's work as a whole), even if it has nothing to do with the story.
And proving that people will try to make anything cute, Cthulhu plushies