To get the trophy that's called "Jaws of Shadow", you will need to perform at least 25 Caragath Stealth kills on those stupid, poor Uruks.
Caragaths can only be found on the Lord Of The Hunt DLC, as far as I know. You can spot them anywhere on the map.
When you start out the campaign, I think that's one of the first missions you do. You tame the Caragath with your Shadow Mount skill, then proceed to find some places that have you kill those fools with stealth kills.
To perform a Caragath Stealth Kill, you have to hold down
and move at the same time. While you are in stealth mode, they can't hear you coming, so just sneak up on an Uruk that's not paying attention and press
to activate the Stealth Kill. The Caragath will growl with rage and pounce on his foe, biting his head off essentially.
Once you do the aforementioned mission, it's simply a matter of hunting down more Uruks and performing those Caragath Stealth Kills until you got your 25th kill. Once you unleash your stealth kill on the twenty-fifth Uruk, the trophy should be unlocked for good.
To help you get a better idea of how to earn this trophy, I've provided a video I spotted on Youtube. All credit goes to richard919 for the video. This video is the best visual aid you'll need to help you with this trophy.
Happy hunting and good luck!