What's the Name of That Book??? discussion

I am not sure if it was YA or not, as I had read the book back in 2006-2009 (the book could have been published before those dates too). I remember the plot focused around this young woman, or teen girl, who had this creature, or being or man who lived in the shadows (not sure which one to be honest) that was trying to take her or the women in her family to give him a child and killed them later on.

A couple scenes I remember was there was a riddle in the book, as well as a scene where a guy friend of the girl finds her in the bathroom at the school dance bleeding after this thing/ person raped her/ attacked her. I think she ended up pregnant later on (maybe not 100% sure).

It was set in the USA, 21st century, female author (?), and I think the story ended in the female character's favor- I honestly cannot remember.

I know the book was something that was very dark and twisted and had been loaned to me by someone else way back when.

Sounds like Impossible

Thank you!! Some old friends and I were trying to make a list of the stuff we used to read for classes/ clubs and for the life of me I spent years trying to remember what it was called!