Source: Epic Level Handbook
You can rend opponents when fighting with more than two limbs.
Prerequisite: Dex 15, base attack bonus +9, three or more hands, Multidexterity, Multiweapon Fighting
Benefit: If you hit an opponent with two or more weapons (wielded in different hands) in the same round, you may automatically rend the opponent. This rending deals additional damage equal to the base damage of the smallest weapon that hit plus 1 1/2 times your Strength modifier. You can only rend once per round, regardless of how many successful attacks you make.
For example, if you wield three weapons simultaneously and you hit with any two of the three weapons in the same round, you would automatically rend your opponent for the appropriate damage.
Special: This feat replaces the Two-Weapon Rend feat for creatures with more than two arms.