Needs review
Source: Masters of the Wild

Description: You know how to hit your favored enemies where it hurts.

Prerequisites: Base Attack Bonus +5, Favored Enemy

Select one of your favored enemies that is normally subject to critical hits. Whenever you attack this type of creature, the threat range of whatever weapon you are using is doubled.

For example, a longsword usually threatens a critical hit on a die roll of 19 or 20 (two numbers). In the hands of a character with Favored Critical using it against a favored enemy, its threat range becomes 17 through 20 (four numbers). If it is also a keen longsword, its threat range becomes 15 through 20 (six numbers: 2 for being a longsword, 2 for being doubled as a keen weapon, and 2 for being doubled again by Favored Critical).