Hastur

Hastur, also known as the  King in Yellow , is a fictional cosmic entity that first appeared in Ambrose Bierce's short story Haïta the Shepherd (1893) and was later expanded on by Robert W. Chambers, H.P. Lovecraft and August Derleth.

History
Hastur debuted in Ambrose Bierce's short story Haïta the Shepherd (1893), in which he was presented as a benevolent god of shepherds.

In Chambers' The King in Yellow (1895), a collection some of which are horror stories, the word "Hastur" is used to describe several concepts such as character and a place.

In Lovecraft's The Whisperer in Darkness (1931) mentioned Hastur alongside many other deities and locations of his own mythology, such as Cthulhu.

August Derleth, a friend of Lovecraft, turned Hastur into a Great Old One, half-brother to Cthulhu and spawn of Yog-Sothoth, who lives in Carcosa. Derleth also presented a rivalry between Hastur and Cthulhu.

In popular culture
The character has had influence in pop culture beyond this:
 * A demon named Hastur appears in Neil Gaiman's and Terry Pratchet's novel Good Omens.
 * As well as it's Amazon TV-show and he served as the possible background villain in the first season of True Detective as the "Yellow King".
 * The Yu-Gi-Oh! trading card game also features a monster with the name "Old Entity Hastorr".