Noir of June

The crown prince, known as the Archmage, is actually a fake with no powers, and behind him is Moa, a real archmage, as his servant. Moa's lifelong love for the crown prince has led her to ignore her failing body and use her magic for him, but when her body eventually breaks down, her return is a terrible betrayal.
In her dying moments, in a dreamlike state of consciousness, someone offers her salvation and revenge on the Prince...
This is servant Moa's Revenge on the Crown Prince

Original Webtoon:
Daum, KakaoPage