"Ave Satani" is a song occasionally featured during an evil or spooky scene. The song has been featured in four episodes so far; "Damien", "Woodland Critter Christmas", "Tsst", and "Britney's New Look".
Background
The song was used as Damien Thorn's theme in "Damien", when his temper caused him to use his satanic powers throughout the episode. The lyrics were changed to "Rectus Dominus, Cheesy Poofs".
The song was also heard during the Woodland Critters "blood orgy" scene in "Woodland Critter Christmas", after they sacrificed Rabbity the Rabbit to Satan. This version closely resembles the original, from the movie the Omen.
In "Tsst", the song is used at the end of the episode to convey that Eric Cartman's evil nature had not completely gone.
The song is later used in "Britney's New Look" by the paparazzi, when they reveal to Kyle Broflovski that Britney Spears needed to be sacrificed, so the corn crops could grow.