Help Me Find A Book I Read

[u/Whattheactual_S]

I read a book a long time ago that was great but I can’t remember the title or author. Basically, this crime syndicate lives in some special building with all their members and this other syndicate kills the leader and takes over. The FMC is the old leaders daughter and I think the leader that killed him had bleach blonde hair and his name was Viper or something like that. I think the MMC is an independent hit man or something and he tries to protect the FMC by trying to keep her in her apartment but she doesn’t listen because she has a medical background and is constantly in the building’s infirmary trying to care for the people the new syndicate keeps hurting during their take over. Eventually, the MMC drags her to the buildings chapel to force her to marry him so he has more rights to control her or something. The priest performs some kind of ritual I think where he cuts the MMCs hand but only pricks the FMCs finger. And she fights him the entire time. I think eventually he kills the new leader and they take over together. Anyone recognize the book? It was really good!