You should have a quest called “The broken shore: investigating the legion”, and if you don’t, you should pick it up from the camp on broken shore. Do that questline and you should unlock the achievement.
And as a note, on the first quest “Investigating the legion” you just have to keep killing demons in the Broken Shore until they drop the quest item. Could take a while (my chars usually got it less in than 5 mins of farming mobs though).