This unlocks once you free Mermund in the rebel camp in Stewark and talk to Lord Renwick afterwards.
At some point you will get the quest The Road to Stewark, this will then lead to the quest Diego in Need.
During this quest you will have to talk to a guard named Winstan and there are 2 ways how the mission proceeds:
(1) you accuse Winstan of lying and fight him
(2) you believe what Winstan says and follow the quest to Liuven
Achievement wise it doesn't matter what you choose. Fighting him is a bit more straight forward. You will find out that Mermund is imprisoned in the rebel camp. So you will have to free him there.
When you speak with Mermund he will give you a new quest: In the Wrong Hands. After some walking you will meet Gorn and then Lord Renwick. When talking to Lord Renwick the achievement unlocks.