UNNAMED MEMORY

"My wish as champion is for you to descend the tower and be my wife." Seeking to end a curse thwarting his lineage, Prince Oscar sets out on a quest that leads him to a powerful and beautiful witch, Tinasha, and he demands a unique bargain: marriage. Though unenthused by the proposal, she agrees to stay in his castle for a year while researching the spell cast upon him. But beneath her beauty lies a lifetime of dark secrets that soon come to light. (Source: Yen Press, Crunchyroll) — Description from AniList community (anilist.co).