Slutty-Princess Diaries

Distol, a prince of Asteria, had passed humiliating days as a hostage in the neighboring country Bartrage, for 8 years. His mother had been also sent to the country, but she ended up dying. Since then, he made efforts to learn fencing and acquiring the knowledge and cultures in order to protect important himself. However, one day, he knew that his sister-in-law whom he had left in his country was elected as a "Holy Lady" in order to seal Satan. Holy Lady would devote her life to seal Satan. Because he couldn't save his loving sister, he felt his weakness.

Then a beautiful girl in black clothes appeared in front of him, and she said she was a witch. In exchange for a strong magic to save his sister, she imposed two conditions on him. They were that he would invade Bartrage to get the secret treasure of the royal family, and that he would kidnap Princess Irina.

(Source: AniDB)