Hastur (The Unspeakable One, Him Who Is Not to be Named, Assatur, Xastur, H'aaztre, or Kaiwan) is an entity of the Cthulhu Mythos. Hastur first appeared in Ambrose Bierce's short story "Haïta the Shepherd" (1893) as a benign god of shepherds. Hastur is briefly mentioned in H.P. Lovecraft's The Whisperer in Darkness; previously, Robert W. Chambers had used the name in his own stories to represent both a person and a place associated with the names of several stars, including Aldebaran.
In Bierce's "Haïta the Shepherd", which appeared in the collection Can Such Things Be?, Hastur is more benevolent than he would later appear in August Derleth's mythos stories. Another story in the same collection ("An Inhabitant of Carcosa") referred to the place "Carcosa" and a person "Hali", names which later authors were to associate with Hastur.
In Chambers' The King in Yellow (1895), a collection some of which are horror stories, Hastur is the name of a potentially supernatural character (in "The Demoiselle D'Ys"), a place (in "The Repairer of Reputations"), and mentioned without explanation in "The Yellow Sign". The latter two stories also mentioned Carcosa, Hali, Aldebaran, and the Hyades, along with a "Yellow Sign" and a play called The King in Yellow.
H. P. Lovecraft read Chambers' book in early 1927 and was so enchanted by it that he added elements of it to his own creations. There are two places in Lovecraft's own writings in which Hastur is mentioned:
I found myself faced by names and terms that I had heard elsewhere in the most hideous of connections—Yuggoth, Great Cthulhu, Tsathoggua, Yog-Sothoth, R'lyeh, Nyarlathotep, Azathoth, Hastur, Yian, Leng, the Lake of Hali, Bethmoora, the Yellow Sign, L’mur-Kathulos, Bran and the Magnum Innominandum—and was drawn back through nameless aeons and inconceivable dimensions to worlds of elder, outer entity at which the crazed author of the Necronomicon had only guessed in the vaguest way.... There is a whole secret cult of evil men (a man of your mystical erudition will understand me when I link them with Hastur and the Yellow Sign) devoted to the purpose of tracking them down and injuring them on behalf of the monstrous powers from other dimensions. —H. P. Lovecraft, "The Whisperer in Darkness"
It is unclear from this quote if Lovecraft's Hastur is a person, a place, an object (such as the Yellow Sign), or a deity (this ambiguity is recurrent in Lovecraft's descriptions of the mythic entities). This is present because the name Hastur is located between a deity's name and a place's name, so his/its identity is unclear.
In "Supernatural Horror In Literature" (written 1926–27, revised 1933, published in The Recluse in 1927), when telling about "The Yellow Sign" by Chambers, H. P. Lovecraft wrote:
"... after stumbling queerly upon the hellish and forbidden book of horrors the two learn, among other hideous things which no sane mortal should know, that this talisman is indeed the nameless Yellow Sign handed down from the accursed cult of Hastur—from primordial Carcosa, whereof the volume treats..."
In Chambers' "The Yellow Sign" the only mentioning of Hastur is:
"...We spoke of Hastur and of Cassilda..."
So, judging from these two quotes, it is quite possible that H. P. Lovecraft not only recognized Hastur as one of the mythos gods, but even made him so recalling Chambers' book.
Derleth also developed Hastur into a Great Old One, spawn of Yog-Sothoth, the half-brother of Cthulhu, and possibly the Magnum Innominandum. In this incarnation, Hastur has several Avatars:
The Feaster from Afar, a black, shriveled, flying monstrosity with tentacles tipped with razor-sharp talons that can pierce a victim's skull and siphon out the brain
The King in Yellow.
Anders Fager's "Collected Swedish Cults" features a Stockholm-based coterie known as "The Carcosa Foundation" that worships Hastur.
Hastur is amorphous, but he is said to appear as a vast, vaguely octopoid being, similar to his half-niece Cthylla.
In the Night Gallery episode "Professor Peabody's Last Lecture," the professor pays the penalty for uttering aloud the name of Hastur.
In the 1990 novel Good Omens (by Neil Gaiman and Terry Pratchett), Hastur appears as a fallen angel and duke of Hell. He is a minor antagonist that tries to bring about the apocalypse along with the other antagonists.
In the 2011 novel Southern Gods (by John Hornor Jacobs), Hastur appears on Earth in the form of a blues musician named Ramblin' John Hastur. The mysterious blues man's dark, driving music - broadcast at ever-shifting frequencies by a phantom radio station - is said to make living men insane and dead men rise.
In Nyaruko: Crawling With Love, Hastur appears as a feminine-looking male that has Wind-based spells and his normal mode can outmatch two aliens. He is also the son of a computer company in space. He also fell in some kind of affection with Mahiro, the male protagonist.
In Unspeakable Vault (of Doom) as "The unspeakable", a yellow octopoid being, wearing yellow robes and a yellow crowned facemask, carrying a rod with a Yellow Sign on it. Referred to as "Hast–He who Is not to be Named", since pronouncing his full name results in a messy explosion.
In the Charles Stross book Annihilation Score the bone violin carried by the protagonist is meant to be played in the unperformed score "The King in Yellow" which summons a linked extradimensional demon.
Hastur, Cassilda, Lake Hali and Carcosa are all referenced by Marion Zimmer Bradley at various points in the Darkover series of science fantasy novels, including eponymously in The Heritage of Hastur (1975). Hastur and Cassilda are semi-mythical founders of one of the great houses of Darkover.
In the 2014 supernatural horror film Mercy, Hastur appears as the main antagonist, seeking to claim the souls of the eponymous grandmother and her unsuspecting family (Also mentioned in Stephen King's short story "Gramma" that the movie is loosely based on)
Popular online game League of Legends features a young fire mage, Annie Hastur, as the daughter of an order of cultists in the 'Voodoo Lands,' and is notable for her dark powers at an early age.
In the 2014 supernatural webseries Carmilla there is a mention about the blade of Hastur, a blade that destroys everything it comes in contact with, as well as mention of a mysterious group known as the "Hastur Monks" or "Hasturmenschen"
The video game Magicka's final boss called 'Assatur, the King in Yellow'. He also appears to be possessing of Grimnir in chapter 7 of the game.
In True Detective (TV series), Hastur is alluded to, but never outright mentioned. "The yellow king" and "Carcosa" appears a few times, hinting towards the presence of Hastur.
Hastur features as one of the evil gods in Wizards of the Coast's collectible card game Hecatomb.
In the trading card game Yu-Gi-Oh!, there is a monster card called "Old Entity Hastorr" that has its name and appearance based off Hastur.
There is a grand tale of adventure about Old Man Henderson in a Trail of Cthulhu tabletop game where Hastur was the primary antagonist.
Hastur appears in his human form in Alan Moore's Neonomicon.
A playing card in the game "Munchkin Cthulhu" features a curse in which the player must say "Hastur Hastur Hastur" for the cards curse to legally take effect.
A unique enemy in the video game Darkest Dungeon called the Collector can appear before the player. It takes immense inspiration from Hastur.
In The Black Tapes episode, "The Unsound", a band is named Hastur Rising.
The video game Ayumi: Enhanced Edition's primary antagonist is the King in Yellow, wanting to offer the crew members to Hastur as a sacrifice.
The "King in Yellow" and the "Yellow Sign" both appear as cards in Darwin Kastle's deck-building game, Cthulhu Realms
Hastur is an unwilling antagonist in Leviathan by Ian Edginton and D'Israeli first printed in 2003. Hastur has his own soul stolen by William Ashbless, who is immortal as a result. In Leviathan Hastur is amoral, and forced into evil acts by Ashbless - in the story finale, out of gratitude for being freed he rescues the ocean liner, and saves 28,000 lives as a result.
