What was the deal with the surgery on the guy they took apart and put back together while a chaos witch danced around him? I got the impression they were removing diseased tissue, but it was all kind of odd.
Long story short, he underwent a specific kind of torture which was slowly killing him over time. To prologue his life and ensure he endured for a few years more - at the most - he became subservient to them, and was oddly regarded as being in a position of honour as a result. As he gradually died bit by bit thanks to their ministrations, he was forced to return to them over and over again for further torture to help make sure he didn't die from the poisons and gradual necrosis overcoming his body. The only thing is it was described in a typically mystic way given that this is the dark eldar we're talking about here, so it's not entirely clear what's happening at first. It was actually kind of a genius move on their part. They sent him back to the Imperium, but he was so utterly reliant upon them that he became an agent within their midst, making it easier for that (exiled) kabal to launch raids on the worlds he governed.
