!. I don't believe either of these were tagged as S8.

Too Late by morganadw:
Sam follows Dean to finally face what was going on between them after his return from Purgatory but misses the clues too late to realize the danger he places himself in until it’s too late. He must try to fight & survive the brutality he endures at Dean’s hands & hopes his brother can fight his own way free or it’ll be too late for them both. Wincest.

The Lines Between Brotherhood and Blood by jasmineisland:
Takes place after Southern Comfort, S8. The Vengeance Specter laid open wounds between the Winchester Brothers. A hunt goes wrong, which only deepens the divide. Just when Dean begins to feel like moving on and splitting up might be the best answer, a Pagan God drags Sam into a mission of absolution that he won't refuse. Dean discovers that his brother has been hanging on by a thread, and the year he was in Purgatory was much worse on Sam than he ever suspected. Cas, Benny, and, worst of all, Crowley see what he has somehow missed -- just how fragile Sam is. With a little help, Dean learns what Sam had been unable to share -- just how far he was willing to go do and what he is still going through, proving to Dean that he is still Sam’s stone number one. Wincest