Start Location: Karn at the entrance to the Cauldron
Note: This quest was IN a pre-order bonus DLC called the "Death Rides Pack" so unfortunately if you were unable to grab that, you will have to miss out on this on unless you buy it as DLC.
Also, this side-quest is NOT required to earn the "Secondary Adventure" achievement/trophy.
This is a three part side-quest that has you entering the first three dungeons of the Forge Lands to find and pick up items specified by Karn. Note that you will be able to spot the plinths that these items sit upon without the DLC installed, however you will not be able to pick up the quest form Karn and therefore will be unable to see or interact with the items themselves.
To make this even easier for yourself, mark the quest as active and the objective marker will lead you right to them!
The Charred Pass
After helping Karn deal with the constructs on the way to the Cauldron, he will lower a bridge for you to enter the dungeon. before heading in have a chat with him and he will ask you to retrieve his silver platter.
Enter the Cauldron and work your way through the dungeon. near the very end you will need to throw a bomb plant across a gap to hit a switch and raise a bridge. On the other side, you will climb the stairs here to fight the boss of the dungeon. Before climbing the stairs, check out the side area to the left to find the platter here. After you are finished with the Cauldron, return it to Karn.
You will meet Karn again in front of the door you need to blow up to continue through to Shadow Gorge, The Fjord and the Drenchfort. Chat with him and he'll ask you to find his helmet. You'll find this in the Drenchfort.
After entering the room with the giant statue, head through the left hand door into the first wing of the dungeon. Climb the stairs here, and rather than going through the door at the top, follow the hallway around to the right until you find another door. Enter here and kill off all the baddies inside. Afterwards check out the southwestern corner of the room for the object. return it to Karn.
Before heading into the Lost Temple, you will find Karn standing outside in front. Speak with him and he will ask you to enter the dungeon and retrieve his compass.
The Lost Temple
After the big fight with the six or so sentinels, climb up the stairs and head into the next room. You will have to go through the left hand door. Wall run across the gap, cruise around the next corner and then wall jump your way up the waterfall to find yourself in a little outdoor courtyard. You'll find the last of karn's items on a pedestal beneath the arch in the centre of the pool of water. Grab it and when you are ready, return to Karn.
For finding all three items, Karn will give you the legendary item Gauntlets of Savagery.