Hastur (The Unspeakable One, He Who Is Not to be Named, Assatur, Xastur, or Kaiwan) is a being usually referred to as part of The Cthulhu Mythos. Hastur first appeared in Ambrose Bierce's short story Haïta the Shepherd (first published 1891) as a benign god of shepherds. Robert W. Chambers later used "Hastur" in his own stories in The King In Yellow to represent both a person and a place relating to the Aldebaran star, and the character Hildred Castaigne is called a "son of Hastur" (in context, probably specifying a place).
See Hastur - The Origin of the Name for more on possible derivations of the name (both real and fictional).
Hastur in the Mythos
In Ambrose Bierce's Haita the Shepherd, which appeared in the collections Tales of Soldiers and Civilians & 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 Robert Chambers' The King In Yellow (1895), the main source of the name's prominence in The Yellow Mythos, "Hastur" crops up in four separate stories – seemingly in irreconcilably different contexts.
- listed with Alar alongside Aldebaran and the Hyades as if they were stars (in The Mask).
- the name of a potentially supernatural servant (in The Demoiselle D'Ys),
- probably the name of a place (in The Repairer of Reputations), and
- mentioned without explanation in The Yellow Sign.
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 is only one place in Lovecraft's own writings that mentions Hastur, but again it is in close association with a reference to the Yellow Sign and, more loosely, with the name Hali. The terms are emboldened here for emphasis:
"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, any more than Chamber's various mentions clarify the name's roots. There is slight elucidation however in Lovecraft's 1925-27 long essay 'Supernatural Horror In Literature' (published in The Recluse in 1927, and revised 1933-34); describing Chambers' story 'The Yellow Sign', he 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..."
The clear implication is that he had come to regard "Hastur" not only as a dark god in the mythos, but as having already been so within The King In Yellow, thereby in effect claiming the latter as the source of this god-Hastur.
August Derleth, publisher and apostle of Lovecraft, 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, shrivelled, flying monstrosity with tentacles tipped with razor-sharp talons that can pierce a victim's skull and siphon out the brain – Joseph Payne Brennan (1976), The Feaster From Afar
- The King In Yellow.
- The High Priest Not to Be Described (last Elder One), an entity that wears a yellow silken mask, although the identification is disputed.
Hastur's form is amorphous, but he is said to appear as a vast, vaguely octopoid being, similar to his half-niece Cthylla.
In Lin Carter's Carcosa Story About Hali, Hastur is named as 'The Dweller in the Depths', 'Spawn of Azathoth', 'Half-Brother of Cthulhu', 'Mate of Shub-Niggurath' (fathering Nug and Yeb upon her) and 'Prince of the Great Old Ones'. He is said to have led the rebellion against the Elder Gods and receives sacrifices thrown into his lake.
Hastur The Unspeakable
This is one of Hastur's more common epithets in post-Lovecraft references, sometimes rendered as He Who Is Not To Be Named, and derives from Magnum Innominandum.
Hastur and classical gods
Interestingly, there was a Roman god of flocks called Pales. Although not directly related to the word pallid, it is possible it shares the same etymology (from PIE pel, grey). Interestingly, Pales was associated with the star Palilicium...
Hastur sometimes appears in literature outside of the Cthulhu Mythos genre of horror.
- Hastur is both a god and a family in Marion Zimmer Bradley's Darkover series.
- In Paul Edwin Zimmer's Dark Border series the mysterious Hastur are guardians and protectors of the world.
- In Good Omens by Terry Pratchett and Neil Gaiman, Hastur is a Duke of Hell who becomes trapped in an answering machine. He later escapes when a telemarketer phones, and promptly devours the entire staff of the telemarketing office (unintentionally spreading a "wave of low-grade goodness" throughout the population).
- The Doctor Who novel All-Consuming Fire equates Hastur with Fenric.
- In the Stephen King short story, Gramma, the titular Gramma invokes Hastur to impregnate her when she is found to be incapable of having a child, and can be made to sleep by being told to "Lie down in the name of Hastur."
- The comic strip User Friendly proposes that Hastur used Usenet as an avatar. Hastur appears in the strip as a sentient blob of very strong coffee made from "Distilled Usenet Bitterness". 
- In the universe of the SCP Foundation The Hanged King of Alagadda is heavily based on Hastur and the King in Yellow.
Extrapolating from August Derleth's epithet for Hastur, "He Who Is Not to be Named", the Dungeons & Dragons Deities and Demigods Cyclopedia supplement suggested that merely speaking Hastur's name brings doom to those who do so.
This idea was later picked up by The Call of Cthulhu RPG role-playing game. It also appears in the PlayStation game Persona 2: Eternal Punishment, where Hastur can be summoned by saying his name three times.
Hastur is the main enemy in the Sega Genesis games Earnest Evans and El Viento. In both games, he's an evil god worshipped by a crazed cult using him to destroy New York City in the 1920s. The heroine of El Viento, Annet Myer, is descended from Hastur's cursed bloodline.
The Call of Cthulhu supplement Delta Green RPG introduced Hastur and his counterpart, the King in Yellow, as manifestations of entropy. This has seen a lot of support amongst fans of the game and original stories – see The Hastur Mythos.
Board games & card games
Hastur outside the Mythos
- Hastur is a family name in Darkover.
- Hastur Sejanus is a character in the Horus Heresy novels from Games Workshop.
- Hastur is the name of a Duke of Hell in Good Omens and Illuminatus! (from which the former derived the role),
- Hastur is the name of the lorespeaker of the Colymer tribe of Sartar in Glorantha in Dragon Past, the in-house Chaosium campaign (article in Wyrm's Footprints).