Riot Gear by onlythisfic on sinful-desire.
Summary: When Dean gets back and reams Sam out for not looking for him, Sam can't bring himself to tell Dean what happened. He thinks Dean will blame him for losing their kid and that it'll be the final straw. Maybe Sam sneaks out to visit the grave and Dean follows, but up to the author how Dean finds out. Cue guilty!Dean and hurt!Sam who has to relive the painful memories from the last year.

And a couple by morganadw on A03. Heed the author warnings for them! There are dark reasons why Sam didn't look for Dean in these.

Too Late
Summary: 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.

Dark Desires
Summary: After checking out a ghost in an old sex club, things go wrong when Sam finds himself on the wrong end of what this club really was used for but it’s not only his life on the line. Both he & Dean must not only survive the ultimate sex curse but the fallout.

'Til It Bleeds Daylight by KatStark on A03.
Summary: Set between S7/8. They defeated Dick Roman, but Dean has been missing for months. Sam is trying to figure out what he's gonna do now when he is kidnapped and held against his will, drugged into submission by a stranger with a nefarious end-game who plays with him emotionally and physically until Sam barely recognizes himself. When they reunite, can the brothers figure out how to put the pieces back together?