Hastur (The Unspeakable One, The King in Yellow, Him Who Is Not to be Named, Assatur, Xastur, H'aaztre, Fenric, or Kaiwan) is an entity of the Cthulhu Mythos.

[8] 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 monstrous powers from other dimensions.

...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..."...We spoke of Hastur and of Cassilda..."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.

"Gramma" was translated into Marathi by Indian writer Narayan Dharap as "Aaji", which served as a basis for the 2019 film Tumbbad, in which Hastur (stylized as Hastar) is the first and most beloved offspring of the Goddess of Prosperity who robbed her of her gold and was stopped while attempting to steal her grain.

Though not named directly, The King in Yellow is mentioned and connected to Carcosa in the first season of the HBO series True Detective.

In the sixth novel in Charles Stross Laundry Files, The Annihilation Score (2017), the protagonist is Dr. Dominique "Mo" O'Brien is compelled by her violin to summon The King in Yellow, with poems referencing Hastur and Carcosa.