Flank Attack (Pathfinder Feat)
From D&D Wiki
Flank Attack [Rogue]
You leap through shadows to attack your foe with deadly strikes that hit them from behind.
Prerequisite: Rogue, Dex 20, Stealthy
Benefit: Every third round, when attacking the enemy from the front a rogue may make a successful acrobatics roll to move behind the foe.