Incisor Fragment

Source
This item drops from King Dred in Drak'Tharon Keep on heroic mode.

Effective Armor Penetration Rating
Assuming this trinket were activated every cooldown, it would grant an effective: 291 * 20 / 120 = 291 * 1 / 6 = 291 / 6 = 48.5 Armor Penetration Rating