The Yellow Sign is a fictional symbol or glyph, first described in Robert W. Chambers' book The King In Yellow. Its nature is unknown, but it seems to possess a strange siren call to the dark world of the King in Yellow and Carcosa, such that those who are exposed to it are doomed.
The King In Yellow
Chambers' book The King In Yellow never fully describes the shape and purpose of the Yellow Sign. The eponymous story offers only that it is "a curious symbol or letter in gold. It was neither Arabic or Chinese, nor as I found afterwards did it belong to any human script". Nonetheless, 'The Repairer of Reputations', another of the stories in the collection, suggests that anyone who possesses or sees – even by accident – a copy of the Sign is susceptible to some form of insidious mind control or possession, by the King in Yellow or one of his heirs. The stories also suggest that the original creator of the Sign was not human and possibly came from a strange alternate dimension that contains an ominous and ancient city known as Carcosa.
Some have suggested that the Sign was represented in Chambers' own illustrations for The King In Yellow. A symbol of a burning, inverted torch (resembling the torch of Thanatos) is shown on the cover design of the earliest editions of the book. While no other evidence exists that Chambers intended this to be the Sign, the imagery serves to suggest the corruption and inevitable destruction which the sign portends. Similar in form to the inverted torch is the thyrsus, the token of Dionysus.
The Cthulhu Mythos
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. Lovecraft refers, in one story, to the Brothers of the Yellow Sign, as enemies of the Migo. In the latter-day Cthulhu Mythos, developed by August Derleth and other Lovecraft imitators, the Yellow Sign is the sign of Hastur and is used by members of his cult to identify one another. In addition, according to many of these works one of Hastur's avatars is known as the King in Yellow. This interpretation is popular among role-players, but many prefer the more vague horror of Chambers' original vision.
Call of Cthulhu
In 1989, Kevin Ross designed a Yellow Sign symbol for the Call of Cthulhu RPG. Ross created the symbol for an adventure scenario entitled 'Tell Me, Have You Seen The Yellow Sign?' in the supplemental book The Great Old Ones. The symbol resembles a yellow triskelion, and is also believed to resemble a tentacled creature, since Hastur is often described as an aquatic tentacled being similar to Cthulhu.
Ross later stated in an interview that the image used is actually a corruption of his original drawing; apparently, Chaosium printed the image both upside-down and backwards. Flipping the image horizontally and vertically reveals Ross' original conception of the Yellow Sign, which resembles a coiled body or tentacle with two tentacles branching upward. Fans have pointed out that this image bears a resemblance to the "Kronos" symbol used by the band Blue Oyster Cult. When asked, Ross admitted to being a big fan of the band, but could not remember if the resemblance was deliberate.
Presumably the mistake occurred because the sign (as familiar to us now) contains what looks like a question-mark, acting as a counterpoint to the question "Tell me, have you seen the Yellow Sign?". Ironically it perhaps makes the Sign appear more fluid, more dynamic, than it would have otherwise appeared.
In Robert Shea and Robert Anton Wilson's Illuminatus! trilogy, the Cult of The Yellow Sign is offered to one detective as a concocted explanation of human history. The cult supposedly worships cthulhoid entities known as lloigor, commits human sacrifice, and has striven throughout history to suppress rationalism. The cult is opposed by the rationalist Illuminati.