Atrocia was the stagename of an Endsville TV show Horror-Hostess, whose TV show was presented in a studio near by both Billy's and Mandy's houses. Atrocia's name was a play on the word "Atrocious", meaning something that is 'frightful', 'evil', 'cruel' or 'monstrous'. Atrocia's character is a parody of Cassandra Peterson's TV show persona of "Elvira: Mistress of the Dark".
Atrocia made her first appearence in Grim or Gregory?, during season 1. In her first appearance, Grim is observed as having a crush on Atrocia, the TV Horror show Hostess who was presenting an all-night Halloween TV Marathon that Grim was watching on Halloween night when Billy and Mandy both demand that Grim accompany them trick or treating. Her character proved to be a popular Halloween costume for the mother of a kid out trick-or-treating that night.
Atrocia was later seen in The Grim Show, announcing that because she had reached the age of 30, she was being retired by her network--which proved to a bad idea for the network executives, as the idea did not go down well with Grim as he was a fan of her show, and went down to their studio to 'raise hell', literally.