Ryuu Ou no Kagehime

Overview

“Ever since that night, I've lived only to kill you." Kohaku was the lone survivor from her hometown, which was sunk and destroyed by the kingdom’s guardian water dragon. She entered the Imperial Place under the guise of a lady-in-waiting, plotting to take revenge on Prince Miaki, who was supposed to be the only person who could control the dragon. However, he seemed nothing like she imagined. And when he caught her, he unexpectedly asked her, “Why don’t you pretend to be my consort instead?” This begins their thrilling contract marriage, to uncover the plot involving the succession to the throne! (Source: LaLa, translated and edited)

Source: AniList