I think I'm still digesting this one a bit, so forgive me if my comments are a little rambling!
This is quite a rollercoaster of a fic. Love the plot twists and mind fuckery, of course. The setting is fantastic; I've never been terribly interested in Durmstrang, but you brought the place to life through some wonderful imagery.
Draco in this is fantastic -- older, with a bit more poise and control than he had as a teenager, but still ultimately ruled by his emotions. The Erotaserum was intended to be both a business venture and a revenge, all of which went awry when those blasted feelings got in the way. Oh, Draco, you're still trying desperately to be the ideal Slytherin, and still failing miserably at it!
There are some scenes in here -- the steam room and the flight sequence, among others -- where the descriptive writing is so smooth that I was really pulled into the setting, and could see, smell, and feel things vividly. Some lovely writing here.