Previous video:https://youtu.be/DW_JhQynkGY
Summary

In Princess Eris's throne room, she and Capper spend time together, with Capper mentioning how similar their values are and impressing her with his sleight of hand. Meanwhile, after Princess Luna has freed Stygian of Eris' nightmare magic, Daybreakerarrives with Trixie and Tempest Shadow. No longer under Eris' control and recognizing Luna's resemblance to her own sister, Daybreaker joins forces with them to defeat Eris and get Luna's magic back.

While waiting for Daybreaker to arrive with Capper's former allies, Eris orders Capper to perform more magic tricks for her, but while doing so, he falls over with Eris' Staff of Sacanas and the orb containing Luna's magic. Fortunately, the staff and orb appear to be undamaged when Capper hands them back to her, and Daybreaker appears with Capper's friends as her chained prisoners. After taunting Luna a little, Eris commands Daybreaker to incinerate them, but Daybreaker refuses to attack her sister. Eris tries to compel Daybreaker to comply using her control collar, but it does not work, the collar and chains revealed to be an illusion projected by Trixie.

Daybreaker explains that her own magic was too suppressed by Eris's to free herself from the collar, so she needed Tempest to do it for her. She also needed a way to remove the collar and get to Eris without her noticing, a solution to which Eris herself provided by capturing Trixie. Now face-to-face with her former master, Daybreaker attacks Eris, but even with Luna, Tempest, Trixie, and Stygian helping her, Eris proves too powerful. Eris prepares to use the Staff of Sacanas on Daybreaker as punishment for her defiance, and Luna jumps in the way of her attack. However, instead of getting blasted with magic, Luna gets hit with harmless bubbles.

Capper reveals that his betrayal of his friends was just a ruse and that, when he fell over with the Staff earlier, he swapped the orb containing Luna's magic with an ordinary crystal ball. Capper shatters the orb on the floor, and Luna's magic returns to her, allowing her to destroy Eris's Staff of Sacanas. Defeated, Eris pleads for mercy, and while Luna does not wish to harm her, Daybreaker engulfs her in fire and sends her flying across the morning sky.

With the threat of Eris eliminated, Luna offers to help Daybreaker return to her original reality, but Daybreaker refuses, instead choosing to stay in Eris's world and occupy the seat of power left vacant in her absence. As thanks for their assistance in freeing her from Eris's control, Daybreaker allows Luna and her friends to leave but warns them to never return. The series concludes with a letter from Luna to Celestia in which she expresses that one cannot force those they care about to make the right decisions, only being supportive and hope that they make those decisions themselves.

Back in their own world, the "Nightmare Knights" close the doorway into Eris's world and go their separate ways, and Luna greets Celestia with a loving hug.