Also you have discovered why Harry Dresden feels a lot better with Michael Carpenter at his side.
Lastly, so what's going to happen after the Death Knight reconstitutes?
Funny you should say that, as my brother has just recently, since we played that adventure, started reading Dresden, and the comparison lept out at him as well. As far as the Death Knight goes, it's job was to protect the Heart of Fire, and the PC's rescued him from the Dao, so there is no bad blood there. It has since been released from it's contract and returned to the afterlife.
On a side note, the Dao fortress they attacked was modeled after a zone in the game Everquest, The Hole. I needed an undead tower in there to stay with the original map, and it all worked out quite well.
Incidentally, the Heart of Fire is actually the leader of the Azer. I'm coming up with a campaign story arc where the Azer are finally making their move to destroy the Efreet, namely conquer the City of Brass. The Azer needed him to run their forges where they are whipping up an army of manufactured Animated Suits, Helmed Horrors, and Iron Golems, something they have been doing for a long time and are about to unleash upon the Efreet. Of course they've enlisted the help of some Gold Dragons to counter the many Red ones the Efreet Sultan has, and several other races who have long felt the lash of the Efreeti tyranny and wish to end their legacy of slavery and pillaging. Plus they built the city and know it's weak points. Not sure exactly how I'm going to pull it off, but a battle for the City of Brass is a pretty big event as it's one of the biggest cities in the planes and home to LOTS of very powerful entities.