There are nine collectables you need to collect and return to Torvin. The trophy 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. 1. Improved Hook with Hinged Movable Bits.
In between the front teeth on the skull.
2. Incline Toxic Death Dispenser.
3. Great Huge Spring Smasher.
4. Fabulously Complex Chain-Reel Drive.
5. Retractable Extra Deadly Bits.
6. Horribly Sharp Chain Razors.
7. Integrated Inferno-Hurling Launcher
8. Big Stoatin' Fire Tank.
9. Wee Enormously Powerful Power-Increaser.
Jump down from here and it will be against the wall. I forget to take a picture before collecting it.