A soul eater, from The Colour of Magic. Represented as a mass with several dozen tentacles. It was about to kill Twoflower and Rincewind when Hrun finally saved them. Has an affinity for the number 8; to wizards this number is dreaded, and they will try to avoid saying it (as well as the word "ate"), substituting "7a" or "two cubed" instead.
It was inspired by the works of H. P. Lovecraft. Substituting the number 8 is likely a reference to the Japanese superstitions around 4 and western superstitions around 13.