From Yen On:
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.

Started on the author's website in 2008, became a web novel on Shousetsuka ni Narou (Syosetu) in 2012 and a light novel in 2019.