To get the trophy that's called "A New Master", you'll have to brand a Captain while in combat.
First off, the Branding ability is learned about through the story, so I don't think it can be missed at all. I'm also unsure if you need to buy it too. But in just case, make sure that you do look in your Abilities menu to see if it hasn't unlocked yet and needs to be bought. If you do, just get the points needed for it until you have enough to unlock it.
Either way, once you do get the ability, it's time to seek out a Captain. First off, find someone that has a green symbol above their head. It can be either an outcast or one of the Uruks that has more information than just their location. This will be easier in dealing with the Captain once you know their weaknesses.
Then it's simply a matter of hunting down the Captain at that specific location and getting into a confrontation with him. Once the battle begins, you can either hack away with your chosen weapon or use one of their weaknesses to your advantage. Either way, whittle his energy down until a green skull symbol appears above his head. That means he's weak enough to be dominated. When that happens, simply face the Captain and press and hold
to not only drain him, but make him yours for sure completely. Keep in mind that you can be stopped during the middle of it if one of the Uruks or a monster nearby strikes you, so make sure the area is cleared out first before branding him. Alternatively, you can also do a Combat Drain, which is performed by pressing both
and
at the same time when facing the captain and that should also work as well.
As an added tip, even if his green symbol does appear above his head and you try to brand him, but he blocks the effort, that's OK. You just need to smack him around a few more times until he's very near death in his health bar before draining him or employing the Combat Brand technique on him.
I have attached a video of how this trophy can be gained if you need a visual aid. All credit goes to PowerPyx for this video.
Have fun and good luck!