Read Unnamed Memory manga
“As long as you’re the master of that sword and I’m a witch, someday you’ll have to kill me for real.”
As a child, Oscar, Crown Prince of Farsas, had been cursed to be childless. At the age of 20, he paid a visit to Tinassha, the strongest witch in the continent, to break the curse. He came back bringing Tinassha along as his protector, and in the one year until his contract with her expired, he got embroiled into her past, and an even greater destiny.
Original Web Novel: https://ncode.syosetu.com/n1488bj/
Official English Translation of the Light Novel: https://yenpress.com/series-search/?series=unnamed-memory-light-novel