Cthulhu is the main antagonist of the Season 5 episode "Prank Call of Cthulhu" in the cartoon series The Grim Adventures of Billy and Mandy.
Prank Call of Cthulhu
He was awoken by Billy and Irwin when they were making prank calls to the Endsville citizens. When he was awoken, he pulls the two boys into his world, but when he realizes how good they were with making prank calls, he gives them the job of sending cursed phone calls to people throughout Endsville, and they were all turned into horrible tentacled monstrosities as a result. When Mandy discovers this predicament, she goes to Cthulhu's world and confronts him. Cthulhu was shown casually playing golf at the time of her visit and he immediately tried to attack her after she had infuriated him. Mandy tricks him into hearing his own cursed message, and he's sucked into the phone himself. He attempted to get through a phone while Grim was on it, but Mandy stops him, and Grim smashes the phone to bits, thereby causing Cthulhu to be trapped in the phone lines for eternity. However, Cthulhu didn't seem as upset over his situation, since he continued to play golf as if nothing had happened.
Grim Adventures of Billy & Mandy