A fic you can’t stop thinking about

Hi guys!

I’m looking to see what fanfic stayed with you for a very long time after you finished reading this. I’m not necessarily asking for your favourite one, but one that you couldn’t stop thinking about for one reason or another. It could have been because it was so well written, or the characters were so spot on, but I’m more curious about the ones that emotionally or mentally left an impact on you.

Let me know please, not just title and author, but also why the story had such an impact on you. Thank you :)