The good ol' King in Yellow, Hastur himself.
When it comes to Lovecraftian abominations, he's one of my favorites because, in the Cthulhu Mythos, Hastur tends to work through artists and creative types, driving them to produce even more maddening creations, and then come under the influence of the Yellow Sign.
The Yellow Sign, if you see it, opens you to be mind-controlled by Hastur, and even moreso if you're a creative type like an artist, musician, or writer.
So....
Have you seen the Yellow Sign?