Kyuuketsu Ouji to Ikenie Hanayome

Meirin An is born into a high-born house, but after losing both of her parents, her home gets usurped by a branch family; she faces mistreatment from them; and they use the presence of her sickly young brother to ensure her compliance. The branch family's daughter, Shuusetsu, takes Meirin's place and becomes engaged to the Third Prince, Tenga Roku. However, Tenga's true identity turns out to be a despised and feared vampire...!?

What awaits the ill-fated young girl is the devotion of a complicated vampire with secrets of his own!? A palace fantasy that begins with a contract marriage!

(Source: Kodansha, translated)