Hastur is called "He Who Is Not to Be Named." A taboo holds that speaking the name three times summons it, and it has long been entangled with the Yellow Sign, the imaginary city of Carcosa, and the play "The King in Yellow."
Hastur originates before Lovecraft, in Ambrose Bierce and Robert Chambers, and was absorbed into the Mythos — a clear example of how the shared world grew through many authors’ hands.