Slam the target with your shield, causing X to Y damage, modified by your shield block value, and dispels 1 magic effect on the target. Also causes a high amount of threat.
Slam the target with your shield, causing X to Y damage, modified by your shield block value, and dispels 1 magic effect on the target. Also causes a high amount of threat.
Shield Slam is a massive direct damage attack which dispels one magic effect, while also causing a high amount of threat. You generate more threat per point of rage with this ability than with Sunder Armor even with only a moderate amount of shield block value. More shield block value makes this the primary threat building attack in the warrior arsenal.
The dispel effect is useful against a large amount of mobs and bosses while also allowing you to take things like Power Word: Shield or Power Word: Fortitude off a player, which is also useful in PvP. The ability offers a good solid chunk of (potentially critical) damage, which is independent of equipped weapon stats, and can even be delivered while disarmed, if need be.
For a pure Prot-spec warrior, this is the hardest hitting attack in your arsenal, so use it whenever you can.
SS Damage = 1.1 * (3046 + .6 * AP) (1.1 assumes you have it glyphed- if you don't, remove it from the formula).
SS Threat = (SS Damage + 231) * 5.0 (Defensive Stance threat modifier).
The damage effect of Shield Slam is applied after the dispelling effect.
Block Value adds one extra damage to Shield Slam per Block Value.
As of Patch 3.0.2, Shield Slam is no longer acquired via talents and is instead trainable from Warrior Trainers. Vigilance replaces it in the talent tree.
As of Patch 3.3.2 the threat from Shield Slam can be calculated as (((Damage + Static Threat) * 30%) * Stance Modifier). The static threat value was not changed, like previously thought.
Shield Mastery increases Block Value, which increases Shield Slam damage.
One-Handed Weapon Specialization increases Shield Slam damage by 2/4/6/8/10% when you have a one-handed weapon equipped (virtually all of the time).
Focused Rage reduces Shield Slam's rage cost by 3.
Sword and Board gives your Devastate and Revenge abilities a 10/20/30% chance to refresh the cooldown on your Shield Slam and eliminate the rage cost the next time you use it.
Critical Block gives your successful blocks a 20/40/60% chance to block double the amount of damage and increase your chance to critically hit with Shield Slam by 5/10/15%.
Gag Order increases Shield Slam's damage by 5/10%.
Patch 3.3.2 (2010-01-02): The damage scaling from block value for this ability now diminishes faster and diminishes starting at a lower block value. The difference should be negligible for players in high-end tanking armor. In addition, the threat caused by Shield Slam has been increased by 30%.
Patch 3.2.0 (2009-08-04): The benefit from additional block value this ability gains is now subject to diminishing returns. Diminishing returns occur once block value exceeds 30 times the player's level and caps the maximum damage benefit from shield block value at 34.5 times the player's level.
Patch 3.0.2 (2008-10-14): Shield Slam is now available to all warriors, starting at level 40 and damage slightly increased.
Patch 2.3.0 (2007-11-13): Shield Slam now always tries to dispel one Magic effect on the target.
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Patch 4.0.1 (2010-10-12): The damage is now based on Attack Power. 
Patch 2.3.0 (2007-11-13): Shield Slam now always tries to dispel one Magic effect on the target. 
