During the week of September 13, 2016 this card was available in a weekly card pack, along with Bounty Stalker and Stinger Boost. You can still earn the contents of that pack through weekly quests.
Splinterbark Vest, in addition to 415pxAbility Armor and 50Max Health also has an Unique Active which, for 6 seconds, makes your basic attacks shred 4 15pxBasic Armor from your targets for 4 seconds. Consecutive hits refresh duration and shred additional 4 15pxBasic Armor.
The shred stacks with 15pxBasic Attack Pen and other sources of Shred.