Overwhelming Critical( Draconomicon, p. 72)
Choose one type of melee weapon, such as claw or bite. With that weapon, you deal more damage on a critical hit.
Devastating Critical (Dr) ,
When using the weapon you have selected, you deal +1d6 points of extra damage on a successful critical hit. If the weapon's critical multiplier is ×3, add +2d6 points of extra damage instead, and if the multiplier is ×4, add +3d6 points of extra damage instead. (Creatures immune to critical hits can't be affected by this feat.)
You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new weapon.