Cruelty of the Kvaldir

Quest does work. First time I killed the naga, the tuskarr got an exclamation point, but ran away while I was still fighting the Kvaldir and I lost him.

Takes a long time to respawn, but if you kill the Kvaldir first, you should be ok.

I just completed it, and it yielded me 300 reputation instead of the noted 150.

Here is the deal:

I ran in twice randomly, started killing stuff, noticed that the Captured Tuskarr Prisoner NPC name was yellow. When I killed the mobs around him and then the naga, he immediately got the quest icon above his head, and ran off faster than I could accept the quest.

So I waited for him to respawn while I was already in the mist. You get the following notification:

Skadir Longboatsman yells: You may have survived for our amusement, but now, you shall die at our hands!

Once you get that notification, the tuskarr will respawn. He is a green icon NPC, and as soon as you kill the naga, he stands there, waits a while, talks a bit, and then pops up the quest icon. He'll sit there as long as you need to get the quest.

You don't have to have this quest to complete the Borean Tundra achievement, but if you really want that quest for some reason, clear the area, let him run off, then wait for him to respawn.

Good luck!