Flanking Strategist Set
(2 items) Adds 1096 Max Stamina
(3 items) Adds 833 Weapon Critical
(4 items) Adds 129 Weapon Damage
(5 items) Adds 345 Weapon Damage to your damaging abilities when you use them to attack an enemy from behind.
Some visual notification about the (5) being active would be nice.
Additional info: The "from behind" of the enemy is a 90° cone, vertically aligned to the centre of its hitbox.