Hotaru no Yomeiri

Satoko appears to have everything—beauty, noble lineage, and the right age for marriage—but suffers from a terminal illness. Her chances of securing a beneficial marriage are threatened when she becomes the target of the enigmatic assassin Shinpei. To protect herself, she urgently proposes marriage to him. However, Shinpei interprets the vow "until death do we part" in a literal and unexpected way.