Wow! You're right--not your usual fare, but you did it in spades! Dark, and twisted, and hot! Guh!
What came through so well was Severus'profound strength, and Sirius' tenuous hold on sanity. Really, really IC. (you can't help but wonder too, if Severus isn't just a little bit glad to be made to suffer. His ideas of atonement can be so masochistic.) Sirius targeting Severus in his insanity was really brilliant, too. He clings to his irrational childhood hatred instead of choosing a more appropriate victim, like the one who put him Azkaban in the 1st place. I think if he had his hands aroung Peter Pettigrew's throat we wouldn't have been so on the edge of our seats.
Thanks for sharing your darker side with us. Really nice, clean writing.