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Hastur is one of the many eldritch gods of H.P. Lovecraft's Cthulhu Mythos. One of the Great Old Ones, he's known as "The Unspeakable One" and also as "He Who Is Not To Be Named." One of his avatars is known as The King in Yellow.
For the trap character from the the gag series Haiyori! Nyaruko-san, refer to the Hastur_(nyaruko-san) tag.