Hastur

"Songs that the Hyades shall sing, Where flap the tatters of the King, Must die unheard in Dim Carcosa Song of my soul, my voice is dead, Die thou, unsung, as tears unshed Shall dry and die in Lost Carcosa"

- Cassilda's Song from The King in Yellow, Act 1, Scene 2

Hastur is a name figuring in several stories and accounts. It notably shows up in the mysterious, macabre play known as The King in Yellow.

Behind the scenes
Hastur is the name of a deity created by Ambrose Bierce and later incorporated in the writings of Robert W. Chambers and other authors, eventually forming part of the Cthulhu Mythos.

In its original appearance in the 1893 short story "Haïta the Shepherd", by Ambrose Bierce, Hastur is a benevolent, bucolic deity worshiped by the title character.

In The King in Yellow
In Robert W. Chamber's 1895 collection The King in Yellow - which contains stories related to the titular (and fictional) play - the name Hastur shows up multiple times, but in different contexts, referring to either a character or a place.

In "The Repairer of Reputations", a deluded man named Hildred Castaigne believes himself to be a descendant of the royal lineage described in the play. Along with his associate, the equally unbalanced "Mr. Wilde", he supports his claims with a document titled The Imperial Dynasty of America, which opens up with the words: "When from Carcosa, the Hyades, Hastur, and Aldebaran". This suggests that Hastur may be a star, given that it's listed alongside Aldebaran and the Hyades.

Through the story, Castaigne identifies himself once as "son of Hastur", and once "King by my right in Hastur", the former being rather ambiguous and the latter suggesting that it's a place. When he puts on the diadem he believes symbolizes his right to the throne, Castaigne narrates that he "thought of Hastur and of [his] own rightful ambition". However, the most significant mention goes as follows:

"Mr. Wilde explained the manuscript, using several volumes on Heraldry, to substantiate the result of his researches. He mentioned the establishment of the Dynasty in Carcosa, the lakes which connected Hastur, Aldebaran and the mystery of the Hyades. He spoke of Cassilda and Camilla, and sounded the cloudy depths of Demhe, and the Lake of Hali. "The scolloped tatters of the King in Yellow must hide Yhtill forever," he muttered, but I do not believe Vance heard him"

- Robert W. Chambers, "The Repairer of Reputations".

In the second story, "The Mask", the protagonist (simply called "Alec") mentions Hastur along with several other references after having read the cursed play known as The King in Yellow. He describes:

"I thought, too, of the King in Yellow wrapped in the fantastic colours of his tattered mantle, and that bitter cry of Cassilda, "Not upon us, oh King, not upon us!" Feverishly I struggled to put it from me, but I saw the lake of Hali, thin and blank, without a ripple or wind to stir it, and I saw the towers of Carcosa behind the moon. Aldebaran, the Hyades, Alar, Hastur, glided through the cloud-rifts which fluttered and flapped as they passed like the scolloped tatters of the King in Yellow."

- Robert W. Chambers, "The Mask".

Similarly, in "The Yellow Sign", the narrator Jack Scott and his friend Tessie Reardon read the accursed play and regret it. Only the vaguest mention is made of the play's content, but it includes the passage:

"Night fell and the hours dragged on, but still we murmured to each other of the King and the Pallid Mask, and midnight sounded from the misty spires in the fog-wrapped city. We spoke of Hastur and of Cassilda, while outside the fog rolled against the blank window-panes as the cloud waves roll and break on the shores of Hali."

- Robert W. Chambers, "The Yellow Sign".

Finally, in "The Demoiselle D'Ys", Hastur is the name of a falconer living in the lost city of Ys in the 16th century. However, there doesn't seem to be anything particularly ominous or even significant about him, or any clue that might link him to the other references besides the name. Hastur here is simply a secondary human character, always seen along with another falconer named Raoul (a name which; unlike Hastur; doesn't appear elsewhere in the book). Furthermore, this story doesn't mention the play at all, so it's unclear whether a connection was really intended.

In later works
After H. P. Lovecraft became familiar with Chambers' work in the 1920s, he included references to Hastur, Yian, the lake of Hali and the Yellow Sign (along with several references to occult elements from his own works and those of Clark Ashton Smith, Lord Dunsany and Robert E. Howard) in his 1931 story "The Whisperer in Darkness". The passage goes as follows:

"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"

The names listed are an amalgam of all sorts of references, including places (Yuggoth, R'lyeh, Yian, Leng), deities (Cthulhu, Tsathoggua, Yog-Sothoth, Nyarlathotep, Azathoth), and other concepts. The name "Yian" might be particularly significant as it refers to the mystical Chinese city mentioned in Chamber's story "The Maker of Moons", which although not explicitly connected to The King in Yellow, does feature the color yellow as a common motif and mentions sorcery, supernatural creatures and the cult of a deity named Xangi. Ysonde (a recurring character in Chambers' prose) explains that "yellow is the symbol of faith [...] in Yian".

In an essay called "Supernatural Horror In Literature", Lovecraft writes his own summary of Chambers' tale "The Yellow Sign", in which he claims the protagonists of the story found that the titular object was "indeed the nameless Yellow Sign handed down from the accursed cult of Hastur - from primordial Carcosa". While a "cult of Hastur" has not been featured explicitly in Chambers' stories, it's possible that Lovecraft was inferring its existence from the above-mentioned references, perhaps including those in "The Maker of Moons", where the color yellow is considered a symbol of faith in Yian, although the people refer to their god by the name "Xangi", not "Hastur". Of course, the idea of Hastur as a deity is also easy to infer given how the name was originally used to refer to the (however benevolent) god that Haïta prays to in Ambrose Bierce's "Haïta the Shepherd". By referring to an "accursed cult" it's possible that Lovecraft's intention was to equate Bierce's and Chambers' Hastur.

Also worth mentioning is that Yian might possibly be linked with Yian-Ho, a "dreadful and forbidden city" located on the Plateau of Leng in the 1934 story "Through the Gates of the Silver Key", a collaboration between Lovecraft and E. Hoffmann Price.

In the mythology created by August Derleth, Hastur the Unspeakable is a Great Old One, son of Yog-Sothoth, half-brother of Cthulhu and equated with the yellow-clad masked figure known as "the King in Yellow" (being therefore the source of the fictional play's title).

Outside the Cthulhu Mythos

 * Hastur was also incorporated into the Doctor Who universe by author Andy Lane in his 1994 novel All-Consuming Fire - also featuring characters from the works of Sir Arthur Conan Doyle. Here, Hastur is equated with the titular entity from the Seventh Doctor's 1989 television serial "The Curse of Fenric".
 * Hastur is an antagonist in the 2000 AD series Leviathan. In the storyline he's depicted as a Judeo-Christian demon instead of an alien god, whose soul was bound into an immortality granting talisman by an occultist in the 12th century, and given to the villainous English knight William of Ashton. Eventually he was built into the power source of the titular luxury liner, and trapped the ship and its passengers onto a hellish ocean for 20 years until his soul was returned. The entity has the power to steal the souls of the dead or the living, in the latter case the body of the victim would be turned into one of his undead minions.
 * The Hanged King (designated as SCP-2264-5) is an enigamitc deity from the universe of the SCP Foundation, that is largely based on Hastur and the King in Yellow.