Genen no Shiro no Navin

Navin, a 13-year-old girl who lived peacefully in a small village in the mountains, lost everything in an instant. The invasion of Adele's army led by Prince Lucian saw her village burned, her friends enslaved, and her mother beheaded before her eyes. Burning with hatred, Navin is picked up by Lucian, her mother's enemy, and forced to swear loyalty to him. She serves him in the closest place possible, in order to kill him one day.

A master-servant relationship connected by murderous intent—the beginning of a full-scale fantasy tale of revenge.

(Source: Kodansha, translated)