Sunder Armor is a warrior ability learned at level 16. It is an instant attack that decreases your opponent's armor by 4% each stack, stacking up to three times. Sundering generates a high amount of threat, making it especially useful for tanking. Not only will it help you keep your opponent focused on you instead of your teammates, it will also increase the amount of damage you and your party members deal.
As well as the obvious and significant armor reduction effect, Sunder Armor is a powerful tool for establishing and holding aggro as a Main Tank. It causes a solid amount of threat, is not situational, and has no extended cooldown, so can be used over and over repeatedly as long as there is enough rage.
For those with the Protection specialization, Sunder Armor is replaced with [Devastate]. The ability applies the same effect, but also has a damage component.
Sunder armor does not stack with the rogue ability, Expose Armor. Since their armor reduction effect is higher, Sunder Armor will not be usable while Expose Armor is on the target, making protection Warriors unable to build threat from sundering. So, be sure to let rogues in your group know not to use Expose Armor.