The key is to make the mastermind look like a loyal ally until the moment you need them to swing. Use someone who can quietly orchestrate the shift—maybe a high-ranking advisor with access to both the throne and the guard. Keep their motivations hidden; let them appear selfless, but in reality they’re pulling the strings. The betrayal should be subtle, a misplaced directive that only reveals its full impact when the final act begins. That way the audience is shocked, but the plan feels inevitable because the mastermind’s influence was always there.