The Grim Adventures of Billy and Mandy

When Nergal comes to tell the guys about his new pizza joint, Grim thinks he can do better by using his Granny's recipe. A pizza war ensues between Nergal's Pizza and Grim's Pizza, with each pizzeria launching TV ads and discrediting each other. Nergal Jr. uses his shape-shifting abilities to infiltrate Grim's pizza and replace Granny Grim's hot sauce with a "ghastly elixir". The result is pizzas becoming monsters and attacking the city, and the citizens developing parasites similar to Nergal. Nergal and his family leave the city just as Grim's sauce explodes.