Armor Piercing reduces the target's effective resistance to attacks by a flat percentage.
Mechanically, this ability works much like a simple damage resistance debuff (e.g Rakish Jab). The difference is that it has no duration and cannot be mitigated with resistances in excess of the resistance cap (75). Even if the victim of an an Armor Piercing attack has more than 75 piercing resistance, the resistance reduction is always a flat percentage.
Armor Piercing is only available to bow weapons. The enchantment is not locked to specific racial vendors, and it can be found in every tier of enchantment in various strengths:
|of the Bull's Eye||15%|
You have 20% Armor Piercing on your bow (of Perforation enchantment), and your target has 75 (capped) resistance to piercing. Your attacks always treat the target as if they only had 55 piercing resistance instead.
The same target returns with 100 piercing resistance (effectively 75 with the cap). You will still damage them as if they had 55 piercing resistance, just like the first example.