Curse of Cthulu is a fictional sequel to the sixth episode of the fifth season of The Grim Adventures of Billy and Mandy, Prank Call of Cthulu.
The inter-dimensional monster, Cthulu, manages to find a way to spread his great demonic powers through his imprisonment inside the phone lines. Doing so, he tricks Billy and Mandy into answering his call. Thus, he turns them both into tentacled monsters akin to himself. However, due to Billy's stupidity of messing with his phone's speaker mode, his happy-go-lucky and good nature gets mixed with both Mandy and Cthulhu, making them happy and peace-loving.
Sharing Billy's goodness, Cthulu continues using his powers to turn everyone in Endsville into tentacled monsters with Billy's happiness and kindness (including Billy and Mandy's parents, Hoss Delgado, Mindy, Sperg, Irwin, Pud'n, Eris, Reginald Skarr, Nergal and his family, and so on). Consumed and influenced by Billy's happiness and kindness, as tentacled monster-turned citizens of Endsville become loving, caring, happy, and enjoy being tentacled monsters. They all wanted to spread Cthulu's power, so they called everyone in the world (including crossover characters) and turn them all into tentacled monsters, and those who don't own any phones are transformed by other ways of communication (payphones, email, cryptic letters, and so on). With no time at all, all the humans on Earth has turn into tentacled monsters and are happy being tentacled monsters forever.
With everyone transformed, Cthulu has all the tentacled monsters on Earth to call the exact same number at the exact same time. Doing so, Cthulhu gets freed from his imprisonment inside the phone lines with everyone bowing at his presence as if he were their king.
Thankful for being freed, Cthulu terraforms Earth into an oozy, digusting, lovecraft-styled world as their new eternal paradise, renaming Earth as Lovecraft. Since that day, everyone lives happily forever as loving and caring tentacled monsters without war, conflict, and hate.