(SPOILER ALERT BELOW!)
(STORY RELATED) *UNMISSABLE*
This is gained after your fight with The Dapper Old Gent in Stonebridge.
Soon after you finish the quest that all the Meister's give you, the Gent will attack the town, this is when you go against him in the causways.
After his defeat you will have him in custody at the Meister Hall, you will have the decision to either spare, or kill him.
After your decision is made the Meisters will vote for the 10th Legion to be allied with Stonebridge and the achievement is yours.
Also remember to go for this during the fight with The Old Gent: