"Demoncism" is the first segment of the forty-second episode of Star vs. the Forces of Evil. It premiered on November 8, 2017 alongside "Sophomore Slump", and is the first segment of the seventh episode in the third season.
Star makes numerous calls to Tom so that they can hang out, but does not get a response. A sweaty Pony Head shows up and unintentionally admits that Tom is doing something. Star gets her to speak up and reveals that Tom is getting a "demoncism" and that he did not want her to know about it. Thinking that Tom is ruining his life or just trying to get back with her, she forces Pony Head to help go and look for him. They end up at the Compact Mirror store to see Pony Head's ex, Seahorse, who seems to be the only person who would know. However, Seahorse comes off as robotic and gives answers only having to do with compact mirrors (Mewni's version of cell phones). After some wrangling, they manage to get a map and locate Tom's location: a spot in the middle of the woods.
Star and Pony Head find that an entire robed community is out there and Pony Head disguises herself to distract everyone. Star goes in the back and discovers Tom where she angrily confronts him. Tom tells her that he intentionally decided to have a demoncism because he realized that he was ruining his own life. After learning that she wanted to be the best princess she could be, he decided that he wanted to make a change and be the best prince. Star believes that he is making a big mistake and leaves with Pony Head. The demoncism stars and the demoncist members prepare the ritual with it awakening something in Tom as he surrounds the camp in blue flame. The members all flee in terror.
Star and Pony Head see the flames and run back to help. As Tom flails mindlessly, only held down by chains, Star jumps up and comforts him until it all ends. Later, Tom wakes up, apparently cured, with Star at his side. The head demoncist arrives and reveals a small bottle with a angry fire demon in it. However, the demoncist tells him that he has thousands more inside and will take thirteen years to remove them all. As Tom wallows, Star tells him that he tried and that is all that counts. Tom decides not to continue the treatment and leaves with Star. Pony Head points out that they are holding hands and look at each other in confusion.
- Eden Sher as Star Butterfly
- Ralph Ineson as Head Demoncist
- Eddie Perino as Seahorse
- Jenny Slate as Pony Head
- Rider Strong as Tom Lucitor
- Dominic Bisignano as Stan
- Fred Tatasciore as Demoncists and Fire Demon
- Tom is revealed to have thousands of little demons inside his body that are responsible for his anger. Only one is removed during his demoncism, and he chooses to deal with his anger issues with the rest still inside rather than remove them.
- When Pony Head is sweating profusely, Star tells her that she is getting sweat on the carpet. Except, Star does not have a carpet.