The Queen of Hearts turned evil because of a humiliating experience during her teenage years at Merlin Academy. Specifically, she was embarrassed by Uliana (Dara Reneé) at the school dance. This humiliation served as the catalyst for her descent into villainy.
To prevent the Queen of Hearts' coup and save Cinderella's life, Red and Chloe attempted to stop Uliana, aiming to alter the past and prevent the event that triggered the Queen's evil transformation.
