dragos last cg is in Ruaden's castle/temple/ big fuck off house in teh swamp.
To unlock it, I THINK you need to have unlocked dragos first cg in that playthrough (tavern at the coliseum), then at some point afterwards while hanging out with him mention that you would like to continue what was interrupted (I think the actual dialouge choice is something along the lines of "what happened that night").
If you did everything right, one night while at ruaden's place, dragon will meet you in a hallway at night and invite you back to his room to finish what you started.