Hastur

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

Hastur was originally introduced into modern writing by Ambrose Bierce, who refers to the name on several occasions, so I am told. In early Cthulhu Mythos writings, the name seems to be that of a place, associated with the stars Aldebaran, the Hyades and the Pleiades, the city of Carcosa and the Lake of Hali. The play 'The King in Yellow' is said to be a ritual aimed at establishing communication with this place. The Yellow Sign is also associated with Hastur.

The idea of Hastur as a creature is due to the interference of August Derleth, one of the H P Lovecraft's heirs. No precise description of this being has ever appeared, and the picture in the Call of Cthulhu rule book of a Salvador Dali-esque melted body is not terribly evocative. It was in the First Edition Advanced Dungeons and Dragons Deities and Demigods book, first printing, that the legend about the name of Hastur first appears. Deriving from other accounts that certain demons can hear their names spoken, and will respond, this legend asserts that anyone speaking the name of Hastur three times will suffer an awful death, purportedly at the hands of the Great Old One himself.

Terry Pratchett and Neil Gaiman use the name in Good Omens to refer to a Duke of Hell, who travels accompanied by another demon named Ligur and causes trouble for the story's protagonists.