Master Assassin attacking enemies from their flank ignores some of their armor resistance. It also increases the stun duration for successful sneak attacks.
This passive's Weapon and Spell Damage is now granted to your damaging attacks against enemies you are flanking, rather than granting a universal amount while sneaking or invisible.
The Weapon and Spell Damage is now a flat 129/258, rather than granting 5/10% of your current amount.
This passive now increases your Physical and Spell Penetration against targets you are flanking by 1487/2974, rather than your Weapon and Spell Damage against them by 129/258.
Developer Comment: While this passive is now in a much better place regarding its accessibility and use cases from Update 27, the class was performing a bit too well in high end PvE DPS. With the adjustment to Penetration, you can expect a little more damage in non-optimized PvE and PvP encounters against high mitigation targets, while tuning back some of their scaling in high end PvE content and specialized gank builds in PvP.