I might be combining fics since I read this a long time ago (like maybe a year ago)... but either I never finished it or the writer never finished it... I dunno, Anywho, it takes place sometime during season 3. Justin is still with Ethan, but Ethan is a drug addict. The Fiddler ends up "loaning" Justin for the night to his dealer. Justin refuses, but then Ethan tells him that the dealer is going to kill him if he doesn't do this, and eventually Justin gives in but ttells Ethan that they're done.
So then this dealer guy ends up taking Justin to some party, and I think this is where I confuse fics....but... it's some sort of exclusive S&M party and the dealer wants to impress the party host, and orders Justin to do something with the host's slave. I think the slave's name was Tyson... or Oscar... it was some boxer's name, I can't remember... they end up fucking each other with a double sided dildo (I think the writer got that scene from the 'Requim for a Dream' movie) while everyone cheers on. At the end, Tyson or whomever finds Justin huddled in a corner crying, and asks what's wrong. Justin tells him the whole story on how this guy is threatening his now ex-boyfriend, etc. etc.
Again, my memory is murky and I might be confusing fics.... but then the Tyson/Oscar guy tells his Master (can't remember his name either) and the man is pissed off, tells his slave to take Justin upstairs to one of the guest rooms to rest.
Later Justin wakes up and stumbles to the top of the stairs and witnesses the party-goers beating the shit out of the dealer, and eventually murdering him.
When Justin returns home with Tyson/Oscar (who drove him to Ethan's apt) he finds Ethan dead, and Tyson/Oscar says something like... "He might have known about the party," or something along those lines to make Justin realize that the party-goers killed Ethan as well.
I can't for the life of me remember what happens next, and maybe it's because the writer stopped writing it? Or maybe it's because I've read so many fan fics that my brain has just morphed a bunch of them together? LOL