First time I ended up killing myself, somehow. I think when his spell is on you as a player you see thru his eyes but also any damage done to him is also done to you. So I nuked myself to death with a big crit!
Second time around I just watched my own HPs very carefully and everything went fine but I think I was lucky NOT to crit as much as my own Hps with a single spellcast since otherwise I'd one-round myself.
When THE DARK died I was still under his spell so my own dots were killing me EVEN THOUGH I DIDN'T SEE THEIR DEBUFFS IN MY OWN SPELL BUFF WINDOW! Had to keep healing myself until I noticed they were no longer running. Because I couldn't tell by looking at the dot icons since they weren't visible. Once I recovered, THE DARK's corpse vanished before I could loot it. "The Dark's corpse disappears into shadow." appeared in chat.
Actually I did get credit for the kill in Hunter Achievement, which is all I wanted.
Strange fight!