Two specific fics one dark!Cas Destiel and the other Alistair/Dean
The first one was a Dark or possibly God!Castiel Destiel fic, where Cas kept Dean captured and tired to get him to join his cause. I vaguely remember that at the end of each chapter when Cas failed or broke Dean, he wiped the slate clean and restarted using a different technique of torture to try to result in a more favourable outcome in the next chapter. I'm 80% sure in was on AO3.
The second was a old fic that was set during Dean's time in hell. It was Alistair/Dean and it featured Bela as a Harpy demon like creature. Another detail is that Alistair's demon form had a pair of deer antlers and at one point Dean realised he had also started to grow horns and a tail as well. Pretty sure this one was on lj.
Sorry for having such a dodgey memory and not being able to give more details!
Thank you! (put dean/omc in the tags because of no Alistair/Dean pairing tag)