Hastur (The Unspeakable One, Him Who Is Not to be Named, Assatur, Xastur, or Kaiwan) is a fictional entity of the Cthulhu Mythos. Hastur first appeared in Ambrose Bierce's short story "Haïta the Shepherd" (1893) as a benign god of shepherds. Hastur is briefly mentioned in H.P. Lovecraft's The Whisperer in Darkness, and Robert W. Chambers later used the character in his own stories to represent both a person and a place associated with the names of several stars, including Aldebaran.
In Terry Pratchett and Neil Gaiman's book Good Omens Hastur appears as a fallen angel and duke of hell.
Read more about Hastur: Hastur in The Mythos