Soren did well to send you out on recon. That's some right good information you have right there.
Trouble is, this just adds more work for us to do. The trolls, confused as they are right now, still would like to kickthe dwarves right out of Coldridge. And I'm certainly not going to let some fiery demon turn our snowy valley into a mountain lake.
The troll leader, his servants, and the fire creature should all be in the cave to the southeast. Take 'em all out.
I'm glad you've returned safely, <name>. While it may not look like it now, you've proven to be a tremendous asset in the protection of Anvilmar.
Here, take this as a symbol of my gratitude.
Notes
Enter the cave and take the left fork. It'll head directly to the back of the cave with both names. Kill trolls along the way. Remember that humanoids are social and will gladly aggro nearby trolls if they're in trouble!