Overwhelming Critical

 
Prerequisite(s): Character Level 21+, Str 23+, Cleave, Great Cleave, Power Attack, Weapon Focus (chosen weapon), Improved Critical (chosen weapon).
 
Benefit: When using the weapon the character has selected, he or she deals an extra 1d6 points of bonus damage on a successful critical hit. If the weapon’s critical multiplier is x3, add +2d6 points of bonus damage instead, and if the multiplier is x4, add +3d6 points of bonus damage instead. Creatures immune to critical hits can’t be affected by this feat.
 
Special: A character can gain this feat multiple times. Its effects do not stack. Each time a character takes the feat, it applies to a different type of weapon.
 
Source: Dungeon Master's Guide, Epic Level Handbook, Draconomicon