Cthulhu Mythos
H.P. Lovecraft read The King in Yellow in early 1927 and included passing references to various things and places from the book—such as the Lake of Hali and the Yellow Sign—in "The Whisperer in Darkness" (1931), one of his seminal Cthulhu Mythos stories. Lovecraft borrowed Chambers' method of only vaguely referring to supernatural events, entities, and places, thereby allowing his readers to imagine the horror for themselves. The imaginary play The King in Yellow effectively became another piece of occult literature in the Cthulhu Mythos alongside the Necronomicon and others.
In the story, Lovecraft linked the Yellow Sign to Hastur, but from his brief (and only) mention it is not clear what Lovecraft meant Hastur to be. August Derleth developed Hastur into a Great Old One in his controversial reworking of Lovecraft's universe, elaborating on this connection in his own mythos stories. In the writings of Derleth and a few other latter-day Cthulhu Mythos authors, the King in Yellow is an Avatar of Hastur, so named because of his appearance as a thin, floating man covered in tattered yellow robes.
In Lovecraft's cycle of horror sonnets, Fungi from Yuggoth, sonnet XXVII "The Elder Pharos" mentions the last Elder One who lives alone talking to chaos via drums: "The Thing, they whisper, wears a silken mask of yellow, whose queer folds appear to hide a face not of this earth...."
In the Call of Cthulhu roleplaying game published by Chaosium, the King In Yellow is an avatar of Hastur who uses his eponymous play to spread insanity among humans. He is described as a hunched figure clad in tattered, yellow rags, who wears a smooth and featureless "Pallid Mask." Removing the mask is a sanity-shattering experience; the King's face is described as "inhuman eyes in a suppurating sea of stubby maggot-like mouths; liquescent flesh, tumorous and gelid, floating and reforming."
Although none of the characters in Chambers's book describe the plot of the play, Kevin Ross fabricated a plot for the play within the Call of Cthulhu mythos. According to Ross' version, the play is set within the fantastical alien city, Yhtill, adjacent to Aldebaran. The plot centers on the members of the city's royal family and their struggle for the throne. Their normal lives are disturbed when they hear of a mysterious stranger who is carried to the city by winged demons (assumed to be byakhee), who openly wears the Yellow Sign and an eerie "Pallid Mask". At the same time, everyone begins seeing a mirage of a city on the other side of the Lake of Hali. The city's upper towers are hidden behind one of the planet's two moons.
The royal family question the stranger, who calls himself the Phantom of Truth, but he only gives cryptic answers and claims to be an emissary of the terrible mythical being known as the King in Yellow, or Last King. At a masked ball honoring the royal family, the Phantom of Truth reveals that his "Pallid Mask" is not a mask, but his true face. Outraged, the queen and high priest torture him to death, but learn nothing in the process. As the Phantom of Truth dies, the King in Yellow arrives from across the Lake of Hali, driving most of the population insane as the mirage-city across the lake vanishes. The King in Yellow informs the royal family that Yhtill has now become the city of Carcosa, under the rule of the King in Yellow. The play ends with the royal family awaiting their imminent doom.
Read more about this topic: The King In Yellow, Influences