Eugene and Varian create a new weapon they call "Project Obsidian", a bazooka that shoots weaponized amber crystals to combat an impending attack from Cassandra, which Rapunzel reluctantly authorizes as a last resort. Meanwhile, Cassandra discovers the missing shard from her mother's mirror and realizes the Enchanted Girl's manipulation, who she finally discovers to be Zhan Tiri. She disguises herself in a magic cloak and takes the form of a handmaiden named Faith in an effort to make amends with Rapunzel. After performing in a play with Rapunzel (which details their battle), Zhan Tiri strips Cassandra of her cloak, revealing herself to the kingdom and causing the guards to attack her. As Eugene and Varian stand down, Zhan Tiri sneaks up and activates Project Obsidian, firing a blast of amber at Cassandra. This proves to be the final straw as she, along with the possessed members of the Dark Kingdom's Brotherhood, unleashes the full power of the Moonstone and declares Corona to be hers. Eugene orders a fall back, and he tells Maximus and Pascal to take an unconscious Rapunzel away from the fallen kingdom. Later that night, Rapunzel wakes up and, to her horror, sees Corona completely taken over by Cassandra, who sits upon the throne as its conqueror.
- Mandy Moore as Rapunzel
- Eden Espinosa as Cassandra
- Hudson D'Andrea as Little Cassandra
- Zachary Levi as Eugene Fitzherbert, Feldspar
- Jeremy Jordan as Varian
- Jennifer Veal as Zhan Tiri
- Clancy Brown as King Frederic
- Bruce Campbell as King Edmund
- James Monroe Iglehart as Lance Strongbow
- Paul F. Tompkins as Shorty
- Susanne Blakeslee as Old Lady Crowley
- Donna Murphy as Mother Gothel
- Steve Blum as Atilla
- Charles Halford as Vladimir
- Dee Bradley Baker as additional voices
- Keith Ferguson as additional voices
- The title plays on an old saying, "Once a/an ____, always a/an ____".
- Cassandra learns the Enchanted Girl is really Zhan Tiri and what Mother Gothel thought of her.
- Zhan Tiri's line, "We're not so different, you and I." is a reference to Dr. Evil's line in Austin Powers.
- Rapunzel stopped Eugene from making a reference to when he almost married Stalyan in "Beyond the Corona Walls" as she keeps blaming her "worthless" journey for losing Cassandra.
- This is the third episode that takes place during the annual Goodwill Festival, the first and second times being "Rapunzel's Enemy" and "Goodbye and Goodwill".
- Cassandra finds the broken piece of mirror that Zhan Tiri threw away in "A Tale of Two Sisters".
- Rapunzel's new handmaiden is revealed to be named Faith.
- Cassandra's line, "You want me to be the bad guy? Fine, now I'm the bad guy." are the exact same words Mother Gothel said when she finally revealed her true nature to Rapunzel in Tangled.
- The play scene is a reference to Star vs. the Forces of Evil Season Four episode "Butterfly Follies".
- List of the characters in the play version of "Destinies Collide" and "Cassandra's Revenge"
- Rapunzel as Cassandra
- Vlad as Cassandra
- Faith as Rapunzel
- Angry and Red as Black Rocks
- Varian refers to himself and Eugene as "Team Awesome" again since "What the Hair?!".
- Adira and Hector attack Corona after they fall under the influence of the Mind Trap Talisman that Cassandra previously acquired in "Race to the Spire".
- Cassandra makes a reference to when Eugene mixed up the bim and dim-berries in "Rapunzel's Enemy".
- Cassandra succeeds in taking over Corona which was Rapunzel's greatest fear in "Be Very Afraid".
- More of Me Is not played in the credits of this episode.
- The scene where Cassandra confronts Zhan Tiri is similar to the scene where Rapunzel confronts Mother Gothel in Tangled.
- Varian is shown to be wearing his outfit, apron, gloves and shoes from season one and at the near end of "Rapunzel's Return" when he is sitting on Fidella before a guard comes to tell Eugene that Cassandra is inside Corona to attack Rapunzel.