Overwhelming Critical( Dungeon Master's Guide v.3.5, p. 210)
Choose one type of melee weapon, such as longsword or greataxe. With that weapon, you do more damage on a critical hit.
When using the weapon you have selected, you deal an extra 1d6 points of damage on a successful critical hit. If the weapon's critical multiplier is Ã—3, add an extra 2d6 points of damage instead, and if the multiplier is 4, add an extra 3d6 points of damage instead. (Creatures immune to critical hits can't be affected by this feat.) SPECIAL: You can gain this feat multiple times. Its effects do not stack. Each time you take the feat, it applies to a new type of weapon.