The Yellow Sign is a symbol first mentioned in Robert W. Chambers' book of horror short stories The King in Yellow (1895). The stories mention the play The King in Yellow, a forbidden work which induces despair and madness in those who read it..
H. P. Lovecraft and many of his imitators were great admirers of Chambers' book and incorporated many of his characters and symbols into their own works. In the latter-day Cthulhu Mythos, the Yellow Sign is the sign of The Great Old One, Hastur, and is used by members of his cult to identify one another.
Camilla: You, sir, should unmask.
Cassilda: Indeed it’s time. We have all laid aside disguise but you.
Stranger: I wear no mask.
Camilla: (Terrified, aside to Cassilda.) No mask? No mask!
—Robert W. Chambers, “The King in Yellow”
Have you seen the Yellow Sign?