Overview
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Sunder Armor is available to warriors starting at level 10. It is an instant attack that decreases your opponent's armor by a certain value, depending on the rank of the skill. Sundering generates a high amount of threat, making it especially useful in parties. Not only will it help you keep your opponent focussed on you, it will also increase the amount of damage you and your party members deal.
Functionality
Normally, Sunder Armor costs 15 rage to perform. It's cost can be reduced by up to 3 using the 
[Improved Sunder Armor] talent.
The resulting debuff on your opponent lasts for 30 seconds and can be applied up to 5 times on a single target. Even after you've sundered a target 5 times, it's advisable to reapply the debuff at least once every 30 seconds to keep it from expiring.
Use
- Cast Cost: 15 Rage
- Cast Time: Instant
- Duration: 30 Seconds
- School and Damage Type: Physical
- Targets/Positioning: Single Target - Front
- Range: 5 yards (Melee)
- Cooldown none
- Stances: All
Talents
[Improved Sunder Armor] (Reduce Rage Cost)
Notes
Requires Melee Weapon Equipped.
High Threat.
Stacks up to five times.
Causes no damage at all.
Rank Table
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Notes
As well as the obvious, and significant, AC reduction effect, Sunder Armor is a great tool for keeping aggro as Main Tank. It causes a good solid amount of Threat, and is not situational, like 
[Revenge], so can be spammed over and over as long as there is enough Rage. (Main Tanks usually have plenty of rage, due to the extra beating they get.)
The threat effect continues to add, even after the fifth stacked Sunder, so Main Tanks should just keep on reapplying it, even after five have hit, stopping only to use [Revenge] when it lights up.
See also: Formulas:Damage reduction for the numerical effects of AC.
Also, rogues should be aware that their 
[Expose Armor] ability does NOT stack with Sunder Armor, and therefore should not be used in an instance in which the rogue is attacking the same target as the Main Tank. The effect will make it impossible for a warrior to sunder and maintain aggro.
For more details about tanking check out the tanking article.
Sunder armor is highly controversial. Some people have reported virtually no change in their damage after 5 sunder applications, and some say their damage has doubled. The DPS of 5 sunders vs. other skill combinations has not been tested.
It must be noted that sunder armor is more useful against those with medium to low armor. The formula for damage reduction works so that the more armor points a character has, the less each powers contributes to damage reduction. For example, 1000 armor will make a reduction of 20%, while 2000 does a reduction of 25%, 3000 a reduction of 27.5%, and so on (these are not actual statistics, just examples to make a point). So reducing armor from 2000 to 1000 is more effective than reducing armor from 3000 to 2000. At level 48 I have 3000 armor and sunder armor, when stacked, can only reduce armor by 1800. Thus, it's not very viable for another warrior to use sunder armor on me, as the change is damage taken wouldn't be sufficient enough to justify its use. Using sunder armor on leather and cloth wearers is probably best.
What is known is that sunder armor can generate a large amount of hate good for pulling mobs off party members when mocking blow or taunt is not available. It should also be used against high level or named mobs. Each party member then receives the benefit of sunder armor.
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