Each time you take damage, you gain 5% of the damage taken as attack power, up to a maximum of 10% of your health.
Vengeance is one of the passive specialization bonuses gained by a character when specializing into a tanking tree at level 10. Its purpose is to help a tank's damage and, thus, threat scale with gear in order to compete with the increasing DPS of end-game content. Vengeance is a bonus provided to all tanking specializations.
Cataclysm Class Preview: Warrior | 2010-04-08 16:01 |Bornakk
Vengeance: This is a mechanic to ensure that tank damage (and therefore threat) doesn't fall behind as damage-dealing classes improve their gear during the course of the expansion. All tanking specs will have Vengeance as their second talent tree passive bonus. Whenever a tank gets hit, Vengeance will give them a stacking attack power buff equal to 5% of the damage done, up to a maximum of 10% of the character's un-buffed health. For boss encounters, we expect that tanks will always have the attack power bonus equal to 10% of their health. The 5% and 10% bonuses assume 51 talent points have been put into the Protection tree. These values will be smaller at lower levels. Remember, you only get this bonus if you have spent the most talent points in the Protection tree, so you won't see Arms or Fury warriors running around with it. Vengeance will let us continue to make tank gear more or less the way we do today – there will be some damage-dealing stats, but mostly survival-oriented stats. Druids typically have more damage-dealing stats even on their tanking gear, so their Vengeance benefit may be smaller, but overall the goal is for all four tanks do about the same damage when tanking.