Cthulhu is a monstrous tentacled entity of chaos who was originally a character in the novels by H.P. Lovecraft. He appeared as the main antagonist in Prank Call of Cthulhu.
At an unknown point, he had been sealed away and placed in a dormant sleep long ago and the only way he could be awoken was by using the Phone of Cthulhu to call him, after which he would rise from the murky depths and destroy the universe.
He had remained in his sleep for countless centuries until he was woken up by Billy and Irwin after they used his phone to prank call him as they were looking for ways to make prank calls without getting caught by caller ID. Cthulhu then kidnapped the two boys and took them to his realm, but having been impressed with their prank skills he decided to employ them and make them part of his cursed prank call staff who were charged with making cursed prank phone calls that would make all who answered turn into horrifying Deep Ones, the traditional servants of Cthulhu.
Mandy and Grim then went in search of the two boys and upon finding out about Cthulhu's dark plan to turn all living beings into Deep Ones that would destroy the world, Mandy decides to put a stop to it and goes to confront Cthulhu and orders Billy to take her to him. They soon find the dark entity casually playing golf in a swamp with his caddie, but Mandy soon puts an end to his fun which greatly upsets the dark creature and he proceeds to attack Mandy. She then uses her cellphone to call the Phone of Cthulhu and tells Cthulhu that the call is for him, upon answering Cthulhu is sucked in by his own phone and pops out on Earth, Grim (who was on the other side as part of Mandy's plan) sees Cthulhu coming out of the phone but is luckily pulled back when Mandy turns off her cellphone and then Grim smashes the phone to bits, leaving Cthulu trapped in the phone lines for all eternity. With Cthulhu gone, the curse on everyone who was turned into a Deep One was removed and all returned to normal. Cthulhu is last seen still trapped in the phone lines but appears to be happily playing golf despite his predicament.
- Cthulhu is normally depicted as green in color with a gigantic and monstrous physique in all of his appearances, but in Prank Call Of Cthulhu he is shown as a purple-colored monster of average height with a humanoid chubby body, a shell on his head and very small wings.
- In the series' intro, a Cthulhu-like entity can be seen amongst the group of villains, however this entity is far more intimidating in appearance compared to the Cthulhu seen in the episode. This may have been a beta design of the character prior to making the episode.