What the title says. Steve in Hydra's clutches, being tortured in awful wronghot noncon ways, including being beaten/whipped/caned directly on his exposed asshole. Followed by agonizing rape once he's so raw, clenched-up, and swollen that they can barely even force their dicks in.

Maybe he's still a virgin and they want to see how tight they can get him before they wreck his hole. (And the humiliation of being touched there at all is so new and unthinkable that it's almost worse than the pain...) Or maybe they've already pounded him until his ass is gaping open and dripping with come, and they whip him until he can hold it in again and then whip him some more. I'm really, really not picky here.

(Bonus: other Avengers forced to watch as their pure, wholesome leader is defiled. Especially if the captors get off on displaying their handiwork.)