Performed 50 Brutal Takedowns.
A brutal takedown is a special type of kill that is performed after an enemy has noticed you. In order to do this you must hold the attack button for the appropriate amount of time when prompted. Keep performing these until you unlock the achievement.
Brutal takedowns can be a bit tricky, as enemies can interrupt the takedown sequence if you're too slow, which may cause a normal attack instead. If you're having trouble, it's recommended to do them while holding a weapon, as this increases the damage done by each hit.