Tell me, have you seen the Yellow Sign?
A symbol of the power of Hastur. It looks like a harmless yellow squiggle, but it appears to squirm when you look at it for a while, and it holds the power to warp dreams. Its appearance often heralds the coming of the King in Yellow. Most of its exposure has come from the stories of Robert W. Chambers and the Call of Cthulhu roleplaying game.
"The Yellow Sign" by Robert W. Chambers (see Cletus' node below)
Encyclopedia Cthulhiana by Daniel Harms, p. 236