This achievement is story related.
In the menu, under Quests > Eltariel Quests, you'll find missions where you can recruit orcs. Since Eltariel doesn't brand orcs, you have to help out these particular ones with a battle or mini-quest they're on to get them to join you. One of the first of these is "The Bow of Morgoth" in Cirith Ungol. Since this is the quest I unlocked the achievement with, I'll explain how to complete it:
*** Spoiler - click to reveal ***In this quest you'll scale a tower, on top of which is a sniper orc with a crossbow. He'll join you if you help him defeat three drakes which an angry Orc captain unleashes. This is fairly easy to do, just shoot one drake repeatedly in the head until it's broken and then Spirit Mount it (hold 
+ 
). Once you've mounted it, fly directly at the other two and have your drake flame them and, once it's built up, shoot fireballs at them. As soon as they are broken, repeat the Spirit Mount procedure until you've dominated all three. Easy. You've helped the sniper orc and now he'll join you and the achievement will unlock.
FYI: completing the bonus objective(s) is not necessary, but is nice for an extra challenge and more loot.
Note: it should go without saying, but if an orc you're trying to recruit dies you won't get the achievement. I don't think any of the orcs I recruited came close even once to dying so this shouldn't be a problem. But do make sure you kill local captains in each region (under the Fallen Blades quests) first. This will make things easier, especially in Cirith Ungol for "The Thrall" recruitment quest.