Ghoulis is a shapeshifting creature called a clutterbumph. It manifests as the absolute worst fear imagined by whoever encounters it, meaning its appearance constantly shifts depending on the observer. The creature possesses no fixed form, instead transforming into whatever inspires the deepest terror in its viewer. Consequently, no two clutterbumphs ever look the same.

Ghoulis serves as Manxmouse's first significant encounter after coming to life. During this meeting, it attempts to frighten Manxmouse but fails because Manxmouse possesses no inherent fears. Despite its typical role as a manifestation of dread, Ghoulis does not harm Manxmouse. Instead, it delivers a critical warning, advising Manxmouse to "beware the Manx Cat," who is destined to consume him. This prophecy establishes the central conflict driving Manxmouse's journey.

The interaction underscores Manxmouse's unique fearlessness; his lack of terror prevents Ghoulis from assuming a terrifying form. After delivering its warning, Ghoulis vanishes from the narrative. The character primarily functions to establish the story's stakes and motivate Manxmouse's quest to confront his fate.