"Butterfly Trap" is the first segment of the fifty-first episode of Star vs. the Forces of Evil. It premiered on March 10, 2018 alongside "Ludo, Where Art Thou?", and is the first segment of the sixteenth episode in the third season.
The trial of Eclipsa begins. To ensure that she tells the truth, Moon brings in a game of truth or pain. Each member of the Magic High Commission asks Eclipsa one question. However, it soon becomes a mock-trial, where every answer she gives is seen as pleading guilty. She was about to be declared guilty when it is revealed that she is allowed to ask them a question as well. She then proceeds to ask the Commission about Meteora, which they initially deny they know anything about. However, the game reveals they are lying and punish them until they reveal the truth: Eclipsa's Mewman husband had her replaced with Festivia, who was actually a peasant. It is then revealed that the whole scenario was set-up by Moon, Eclipsa, and Star to force the Commission to reveal the truth. Outraged by the Commissions lies and the fact that they are not the true royal family, Star sets Eclipsa free and runs off.
- Eden Sher as Star Butterfly
- Grey DeLisle as Moon Butterfly
- Esmé Bianco as Eclipsa Butterfly
- Zosia Mamet as Hekapoo
- Carl Weathers as Omnitraxus Prime
- Kevin Michael Richardson as Rhombulus
- Rhombulus accidentally admits that he has a crush on Queen Moon.
- It is revealed that Queen Festivia was originally a peasant girl before being transplanted onto the Mewni throne by the Magic High Commission. This means all of Festivia's descendants, including Moon and Star, are not true-blooded members of the Butterfly family.
- It is revealed that the game, “Truth of Punishment”, from “Sleepover“ is actually called the “Box of Truth”. It’s also not supposed to be a game, it’s used for trials and interrogations.