What is "hastur"

Hastur ( The Unspeakable One, Him Who Is Not to be Named, Assatur, Xastur, H'aaztre, or Kaiwan ) is an 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; previously, Robert W. Chambers had used the name in his own stories to represent both a person and a place associated with the names of several stars, including Aldebaran.