You can begin this questline by talking to Ost Odura in the Gorhart Inn (the northern building by the fountain). He will start you on a chain of quests towards completing this achievement. I will list the quests below and (eventually) provide a more detailed walkthrough.
Warsworn Quests:
- The Commendation
- The Road Patrol
- Rally Cry
- The Heart of Sibun
- Lock and Key
- House of No Doors
- The Isle of Eamonn
- The Mystic Hammer
- The Mountain Prison
You will unlock this trophy after you complete the quest "The Mountain Prison" by returning the Hall of the Firstsworn for a presentation.
Also note that this faction questline is a great place to net the trophy "Niskaru Slayer".