Until You Disappear

The demon catcher girl who couldn't use the spell was rescued by the strongest monster, but she heard him say, "You are the only one who can kill me" to conclude a contract that ends with death. What kind of connection is hidden behind the two, and how should this agreement be fulfilled? [Original Webcomic on KuaiKan](https://www.kuaikanmanhua.com/web/topic/2179)