The primary antagonists in Chapter 6, a gentle spin on Murder on the Orient Express, are the Three Shadows, now Beldam, Marilyn, and Doopliss. Notably, Beldam threatens Vivian specifically: “Normally, this would be where I finally deal with you and that homely traitor, Vivian…” Doopliss impersonates a celebrity as part of a plot to bomb the train Mario is riding. Strangely, the player does not actually fight the group, who simply run away when thwarted and later have a comical scene where they again screw up a scheme. (As a kid, I thought Beldam sends the Smorg monsters that subsequently attack the train as her plan B, but in fact the script never even implies this.) Note that the writers preemptively avoid having Vivian be involved in the more villainous, albeit still ridiculous, train bombing, allowing her redemption to be simpler. While the player can use any party members, Vivian is depicted as proactive about the wacky detective hijinx and thwarting Beldam as the others.
The Three Shadows only return in Chapter 8, the finale, where they attack Mario’s party inside the Palace of Shadow where the ancient world-conquering demon is sealed. Beldam specifies she intends to kill Vivian: “Ready for the end, my dears?” says Beldam. “You and that traitor Vivian have a date with eternity!” If Vivian is the active party member, now empowered, she inverts Beldam’s perpetual threats of punishment: “Today, Sis…I’m going to punish YOU, you hear me?” While any party member could be active instead, this optional dialogue completes Vivian’s arc: no longer a submissive but villainous victim, she is actively standing up to her sisters. The fight is much tougher than in Chapter 2, but Beldam, Marilyn, and Doopliss are left dazed on the ground with cartoon stars orbiting their heads.
At the climax, Beldam, Marilyn, and Doopliss reappear to deliver a surprising twist. Far from a joke villain, Beldam is the mastermind behind the entire plot to unseal her master, the demonic conqueror the Shadow Queen, and re-enslave the world. If Vivian is the active party member, her surprise and confusion indicate that she was unaware this is what her sister was up to. This has a number of unexplored implications. For one, Beldam must be centuries old, which raises the question of how old Vivian is.
When she suspects they have to kill Princess Peach, whom the Shadow Queen is using as a host body, Vivian shows a darker side. She believes that after Peach is dead, she herself will be able to be together with Mario: “I feel bad for the princess, but that queen must fall before us! And when she does, you and I can… Well, anyway, let’s take this fight to her!” In a long final boss fight, Mario and his friends, with the power of the goodwill of all the people they’ve helped on their journeys, are able to break through the Shadow Queen’s powers, rescue Peach, and kill the demon. Beldam and Marilyn run away.