It was also the title of a roleplaying magazine.
The Encyclopedia Cthulhiana says...Edit
Bargain made with Hastur by unsuspecting magicians. The magician's side of the bargain may differ, but the end result is the same: the Old One possesses his or her body upon death, taking on a humanoid form with boneless arms. The Unspeakable Promise is also said to bind Shub-Niggurath in some manner.