"Ghost of Butterfly Castle" is the second segment of the sixty-fifth episode of Star vs. the Forces of Evil. It premiered on April 7, 2019 alongside "Princess Quasar Catepillar and the Magic Bell", and is the second segment of the ninth episode in the fourth season.
Moon revisits the Butterfly Castle ruins to discover an item she left behind, but she is soon surprised.
Moon is relaxing outside her yurt home when she is interrupted by her fellow Mewmans who are arguing over using the river for their laundry. She stops them and attempts to create a schedule for them, but hey continue to bicker over other things. The Mewmans insist that they still follow her over Eclipsa, despite the fact that their loyalty is purely based on blind prejudice, and want her to give them orders. Eclipsa, frustrated, decides to head back to Butterfly Castle to get her day planner. River warns her that the castle is supposedly haunted, or cursed, and that she should just "be selfish" for once. Moon leaves anyway and upon entering the kingdom discovers it in total disrepair.
She makes her way through the castle to find much of it destroyed due to "anarchy" and discovers that many of the tapestries of her family have been repaired. After finding her planner, she spots a ghost who turns out to be Mina who has been trying to protect the place from looters. She reveals that she had attempted to poison Eclipsa through the Yada Yada Berries she was growing and that she has been plotting a way to overthrow her. She asks Moon to restart the "Solarian Soldier Project" so that they can take the castle by force, adding that she is the only successful soldier from the project and wants more like her.
Moon tells her that the project was from a dark time in Mewni history and that even though she is glad that she is loyal to her, she no longer finds it necessary to fight back. Feeling betrayed, Mina accuses her of being a traitor and announces that she will revolt against Eclipsa herself and leaves. Moon, annoyed, callously gives her planner to the Mewmans and tells River about the revelation. He asks her if she will do something about it, but she tells him no and that he will take it easy like he suggested earlier.
- This episode mentions Solaria Butterfly's magical super-soldier creation project, the "Solarian Program", that was first mentioned in Star vs. the Forces of Evil: The Magic Book of Spells.
- The community that Moon and River established in "Down by the River" has now grown to over a dozen Mewmans.
- Mina Loveberry is revealed to have been guarding Butterfly Castle and restoring its tapestries since it was abandoned.
- She is also revealed to be the one who tried to turn Eclipsa to stone in "Yada Yada Berries".
- She is also revealed to be the last remaining survivor of Queen Solaria's "Solarian Program".
- This marks as the only paired episodes to not feature Star Butterfly and Marco Diaz.
- June 7, 2019 (Southeast Asia)