Fortress by Glasslogic
Seven years ago, Dean Winchester paid his due to the Crossroads Demon and fell into Hell. Grief almost killed Sam, a demon blood addiction came closer, but he rid himself of demonic attachments and spent years building himself a prison where no demon can set foot, determined to take himself out of the apocalypse game that had crushed his family. But each year on Earth is an eternity in Hell, and now Dean is back. Angry, powerful, and demonic; he has a plan to wreck vengeance on those who destroyed his life. A plan that will shake the foundations of the Pit and send shock-waves into Heaven. Dragging a reluctant Sam in his wake, will Dean manage to pull it off before the combined legions of Hell get their act together enough to stop him?
(NC-17 for dub-con, blood-play, language)