Given a reference text about "Ghouli" tell me what happens in the episode.

"Ghouli" is the fifth episode of the eleventh season of the American science fiction television series The X-Files. The episode was written and directed by James Wong. The tagline for this episode is "You see what I want you to see". Though not originally billed as a mythology episode, "Ghouli" helps to explore the series' overarching mythology and serves as the second of three mythology episodes of the season following the season premiere.

The show centers on FBI special agents who work on unsolved paranormal cases called X-Files; focusing on the investigations of Fox Mulder (David Duchovny), and Dana Scully (Gillian Anderson) after their reinstatement in the FBI. In this episode, A pair of teenage girls attack one another, each believing the other to be a monster, known as "Ghouli." Mulder and Scully find that their investigation leads back to their long-lost son, William.

In the episode "Ghouli", Fox Mulder and Dana Scully investigate a pair of teenage girls that attack one another believing the other is a monster.