Okay, I can only remember the starting scene of this story, and it's been bugging me for days. Harry is captured by the Dark Lord, and Voldemort offers Harry as a "prize" to his loyal. Severus is there and they are allowed to fight for the right to go first. Lucius is VERY interested in Harry, but Sev manages to kick Luci's rear. He claims Harry.
He knows the Order is coming, but is unable to delay long enough. I *think* Harry realizes that they both have no choice in the matter and so Severus takes Harry in front of everyone and then the Order shows up and gets Harry the hell out of there.
I know there's a scene further in where Severus confronts Harry with what he did to him. I want to say that Harry isn't *as* rattled as one would suspect him to be, but it's gutting Severus to see him in class, knowing what he's done.
I'm hoping this will ring a bell and I'll feel like an idiot for not knowing the title or author right off.
Found!
Absolution by Rushlight