Hold (RT) to Crouch and enter Stealth Mode. This makes you harder to detect. While in Stealth Mode, unaware enemies can be eliminated using Stealth Attacks. Entering long grass undetected will automatically activate Stealth Mode. An outline of the Wraith will display the last known position where enemies lost sight of Talion.
While crouching and unseen, press (X) to silently kill an enemy.
Stealth Kill - Ledge:
Press (RT + X) while hidden and hanging from a ledge directly below an enemy to have Talion pull that enemy off the ledge to their death.
Press (RT + Y) (Requires Upgrade) to brutally stab an enemy, increasing your Hit Streak to enter combat and terrifying Talion's foes.
Press (RT + X) while above an enemy to dive on them from above, knocking them down. This ability can be upgraded to kill Talion's enemies by unlocking "Strike From Above".