OK, since no one was really explaining HOW to do this quest, here's a guide for those who just keep dying.
Thalgan Blightbringer is a nasty-looking, massive Scourge monstrosity just north of the questgiver, Avatar of Freya. You should be able to see him from where you're standing, unless you have your video settings turned down. If not, go north just a little bit and look under the triangle formation of the floating mini-necropolises burning with fire. (If he's not there, wait for him to spawn. He pops up from the ground)
Aggro him any way you choose, but here's the skills you should look for.
He *really* likes to do a "rain of fire" sort of spell. Obviously, fire will rain down on the ground around you. If you're not watching for it, and you're stuck in it, it can actually damage you pretty good. He stands still while casting it, so just move out of the way and put a healing spell on yourself if needed. Mostly it's just really annoying, because he casts it the most.
The spell you want to reflect back at him is actually hard to spot if you have no idea what you're doing or have never done this quest before. He brings his hands close to his right side and orbs of shadow form around his hands. Wait a second or two, then use the reflection the Avatar of Freya gave you. If you don't time it right, it CAN and WILL kill you. Took off a huge chunk of health when I was in catform.
Usually takes about 3 reflections to kill him, at least for me since I'm a melee. Had to heal a few times, and died the first time I tried, but it's a pretty easy quest once you know what to look for.
Then, just head south a few steps to Freya and turn in the quest.
*EDIT* The screenshot someone uploaded has an example of Thalgan casting the "deathbolt". To my screen, it blends in pretty good with the background, especially if you're a big hulking bear blocking half the guy's body. (lol)