I recently read The Georgia Incident by MarquessaS, and got me in the mood for stories that were like that, where Sam used his demonic powers on Dean causing him great injury, whither it be permanent, fixable or a deathfic.
My second search is a story where Dean is the real antichrist, not sure if Sam is playing the role to take the heat off of Dean, or not, that's about all I know if it, Hope someone out in the spn universe recognises it. An upfront thanks to all how reply.