There are nine collectables you need to collect and return to Torvin. The achievement pops when you talk to him after the last collectable has been found. You can fast travel to his home on the western section of the map, where you start the DLC.
I have a feeling that I have mixed some of the names up, so please comment if you find one. All the images are correct though.
Names should be fine now, thanks to Warboy925
1. Improved Hook with Hinged Movable Bits (Catapult). In between the front teeth on the skull.
Note: the map image below is a replacement as the original has strangely vanished. This link won't format either. You can click the URL and it will open a new tab, or go to collectable number 2 for the same map. Ignore the player icon and head to the skull in the south, next to the border. You can't miss the skull.
2. Incline Toxic Death Dispenser (Wormrot Oils).
3. Great Huge Spring Smasher (Mighty Shield).
4. Fabulously Complex Chain-Reel Drive (Grapple Pull).
5. Wee Enormously Powerful Power-Increaser (Chain Kills).
6. Horribly Sharp Chain Razors (Cyclone).
7. Retractable Extra Deadly Bits (Fatal Counter)
8. Integrated Inferno-Hurling Launcher (Numenorean Fire).
9. Big Stoatin' Fire Tank (Blazing Shield). Jump down from here and it will be against the wall. I forget to take a picture before collecting it.
Here's a video if you need any additional help: