Alright, so I read a fic awhile back that I could swear was on Ao3, but I can't locate it again. It's season 7 and focused a lot on Sam and Dean. I know for a fact that it is not Tangible. Here's what I remember:

1. Sam's hallucination of Lucifer was able to interact with the world around him as it wasn't just a figment of his imagination and was screwing with him.

2. There's a scene where Sam and Dean are talking about everything going on and Lucifer breathes onto the backseat window to draw on it and Dean reacts with something along the lines of please tell me that's not really happening.

3. They needed to get their hands on Lucifer's trident to defeat the leviathans. There were some original characters (I think a goddess of sorts?) that helped direct them and were going against the leviathans as well.

4. Somebody helped them do a ritual. They were going to search heaven and hell for the trident, but somehow Sam screwed up and he and Dean went in two separate directions.

5. Sam wound up in hell because of the ritual and ended up breaking the cage open in needing the trident.

6. Not sure, but I think it might have been Sam/Lucifer?

Any help will be greatly appreciated!