XOR first appeared as a secret addition in Five Nights at Freddy's: Ultimate Custom Night, released in 2018 as a free standalone game by Scott Cawthon. She is a dark variant of Dee Dee and appears randomly to summon an extra character from a hidden roster, including rare animatronics. Clips of her lines spread through FNAF wikis, YouTube playthroughs and soundboards alongside the FNAF Jumpscare Scream, becoming shorthand among fans for a run getting unexpectedly harder and for the lore hunting around secret characters.