Unnamed Memory, Vol. 1 (light novel): The Witch of the Azure Moon and the Cursed Prince — Kuji Furumiya

“My wish as champion is for you to descend the tower and be my wife.”
Climbing a deadly tower, Oscar seeks the power of its master, the Witch of the Azure
Moon. He hopes her incredible magic can break a curse that will kill any woman he
takes for a wife. When the prince sees how beautiful Tinasha is, though, he has a better
idea -- since she's surely strong enough to survive his curse, she should just marry
him instead! Tinasha isn't keen on the idea, but agrees to live with Oscar in the royal
castle for a year while researching the spell placed on the prince. The witch's pretty
face hides several lifetimes of dark secrets, however -- secrets that begin resurfacing…