Penance is the Play by Maeglin Yedi
http://www.etc.slashcity.net/maeglin/viewstory.php?sid=136 Summary: Harry wants revenge. Harry has a plan. But is Harry Slytherin enough to avoid his own trap?
Genres: Romance, Angst, Slash
Rating: NC-17
Warnings: Dubious consent, Coercion/Manipulation
Complete in approx 70,000 words
I've just spent a few hours reading this from start to finish. I was thoroughly drawn in right from the beginning, even though it starts with, as the warning says, slightly dubious consent. Has a great theory on Dark Arts and Dark magic, too.
The author's site has some 50 stories, including some from other fandoms like LOTR, as well as some essays relating to fanfiction. A great read.