By some accounts, one of the Great Old Ones of the Cthulhu Mythos who seems to take offense at those who speak its name. It 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.

By other accounts, Hastur is a force of entropy embodied by the King in Yellow and centered in the lost city of Carcosa and in the Lake of Hali. By nearly all accounts, Hastur was created by Robert W. Chambers and later incorporated into some of the stories of H. P. Lovecraft.

"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