Needs review
Source: Way of the Ninja

Description: You can inflict deadly blows to a target's vital organs using a knife

Prerequisites: Weapon Focus, Sneak Attack +1d6

When using a tanto, aiguchi, or dagger (effectively the same sort of weapon) against a humanoid (or monstrous humanoid) opponent, your critical threat range is increased by 1 and the weapon inflicts 1d6 damage instead of 1d4.