A god from the Cthulhu Mythos, originally created as a benevolent presence by Ambrose Bierce and later incorporated into the stories of Robert W. Chambers before being added to the pantheon of the Great Old Ones by H.P. Lovecraft and the other writers associated with the so-called Lovecraft Circle.
Hastur is often depicted as a tentacled, slug-like monstrosity with a somewhat sullen and jealous attitude. It is, however, unhealthy to point this out in Hastur's presence.
Sometimes known as "the Unspeakable," which may mean that he's particularly evil or may mean that his name should not be spoken.
Hastur is also a force of entropy embodied by the King in Yellow and centered in the lost city of Carcosa and in the Lake of Hali.
"Haita the Shepherd" by Ambrose Bierce
"The Return of Hastur" by August Derleth
"The Whisperer in Darkness" by H.P Lovecraft
Encyclopedia Cthulhiana by Daniel Harms, pp. 94-95
Delta Green: Countdown by Dennis Detwiller, Adam Scott Glancy, and John Tynes, pp. 200-213