Unlocking the quest
You can unlock this quest during the main quest [To Move a Mountain]. Go to the entrance to [The Lost Temple] to find Karn on the left. In the conversation, select One more favour and you'll be asked to retrieve a compass belonging to Karn from The Lost Temple.
Find Karn's Compass
The item you're looking for is Karn's Compass and you can find it during a visit to [The Lost Temple] in the main quest [To Move a Mountain]. Get to the small area with a garden, which is not long after the battle with a few Construct Sentinels. Examine the central alcove and you'll find what you need.
Bring the Compass back to Karn
It's best to start looking for Karn only after completing the main quest [To Move a Mountain], which is after leaving [The Lost Temple]. You should find him in the [Forge Lands] (adjacent to Tri-Stone). Trigger a conversation and select I've found it.... The reward is an additional weapon Gauntlets of Savagery.