The material below is Open Game Content from a 3PP Source
Stunning Critical (Combat, Critical)
Your critical hits cause opponents to become stunned.
Prerequisites: Critical Focus, Staggering Critical, base attack bonus +17.
Benefit: Whenever you score a critical hit, your opponent becomes stunned for 1d4 rounds. A successful Fortitude save reduces this to staggered for 1d4 rounds. The DC of this Fortitude save is equal to 10 + your base attack bonus. The effects of this feat do not stack. Additional hits instead add to the duration.
Special: You can only apply the effects of one critical feat to a given critical hit unless you possess Critical Mastery.