I was re-reading the 'Redemption Road' series when I remembered this fic existed and I adored it. Here's what I recall: it is definitely an OLD story (at least 5 years ago), it was a 'Cas goes to Hell to save Dean' pre-start of season four fic, and it isn't any of the stories mentioned in THIS: http://spnstoryfinders.livejournal.com/8710379.html similar request. I'm not sure if it was Destiel slanted or not - but it was more about the journey through Hell then the torture stuff. I also think it started with Cas and Uriel (maybe Balthazar?!) discussing the plan, though in the end it was just Dean and Cas. Last but not least - I'm fairly sure it was NOT on fanfiction.net. Sorry I can't be more detailed and any help would be much appreciated. :)