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In "Weirdmageddon Part 1," The Horrifying Sweaty One-Armed Monstrosity is one of the monsters that inhabits Gravity Falls during Weirdmageddon. He encounters Dipper as he attempts to break in to the Gravity Falls Mall in search of his friends. He demands that Dipper climbs into his mouth, which Dipper ignores in terror, much to the Monstrosity's frustration. He also appears during the episode's credits, still desperate to convince anyone he can to climb into his mouth.
- Louis C.K., the creature's voice actor, improvised most of his role during the end credit scene of "Weirdmageddon Part 1" where he says, "I gotta call my mother." In fact, there is nearly 30 minutes of him improvising different ways the monstrosity could beg someone to get into his mouth.