EDIT:Thanks to
Warboy925 for pointing out that this a mission in the DLC
Lord of the HuntAfter completing the mission entitled “
Unwarranted Aggression”, Talion will remain mounted upon a dominated Wretched Graug. At this time, begin traveling across Nurn with Wraith Vision activated until the red outline of a Uruk Captain becomes visible. As soon as you spot your target, tap the right thumbstick in to mark the Captain and start heading over to his location.
When you are near the target, tap
to initiate the Graug’s Bite Attack. This will cause the Graug to sweep its hard across the battlefield, pick up the Captain, and devour him whole, thus unlocking the "
Nom Nom Nom!" achievement.