To earn this achievement you must complete the Main Quest of the Orsinium DLC. This includes 9 main quests and 3 side quests (these latter three are completed as a byproduct of the main quest 'In the Name of the King').
The main quests are as follows:
1. Invitation to Orsinium
2. For King and Glory
3. A King-Sized Problem
4. In the Name of the King
5. To Save a Chief
6. The Anger of the King
7. The King's Gambit
8. Blood on a King's Hands
9. Long Live the King
The three side quests that you must complete as part of 'In The Name of the King' are (in no particular order):
1. Quarry Conundrum
2. The Hand of Morkul
3. A Question of Succession
Detailed guides for each quest can be found through the following:http://en.uesp.net/wiki/Online:Wrothgar
Once you have completed all of the aforementioned quests, the achievement will unlock!Note
: The only quest I think is particularly difficult is the 'The Anger of the King' whereby you must complete a timed trial and a boss battle towards the end of the mission.
I would recommend trying to complete the timed trial as fast as possible, only eliminating the enemies who are essential to remove in order to progress. For the boss battle at the end, it is much easier to defeat Talviah by herself than the group of Vosh Rakh council members, so opt for 'sacrificing Talviah' when you reach that point.
Whilst the Talviah boss battle remains difficult, I recommend:
1. Keeping your distance during Talviah's long-range attacks whenever possible
2. Using an ability that allows you to drain her health whilst she periodically attempts to heal herself
3. When entering close-quarters combat, run around her and chip away at her health as opposed to standing still and launching attacks from one direction.