Avenger's Shield is a paladin ability learned at level 10 for those with the Protection specialization. It is a ranged attack that will jump to 2 additional nearby targets, silencing them for three seconds.
In PvP, this ability is best used as an interrupt to spellcasters, due to its interrupt ability and 3 second silence. Combined with [Grand Crusader] and [Rebuke], the protection paladin can keep a spellcaster bereft of spells for significant bursts of time.
The ability is a reference to Captain America (who is a member of the superhero group known as The Avengers), and his ability to ricochet his shield off of multiple targets and return to him.
Patch 4.1.0 (26-Apr-2011): Avenger's Shield can no longer be blocked.
Patch 4.0.6 (8-Feb-2011): The Silence debuff from Avenger’s Shield was using the Dazed visual effect, and its Daze debuff did not have a visible effect. This has been remedied.
Patch 3.3.3 (2010-03-23): Once again does not strike secondary targets affected by Repentance.
Patch 3.1.0 (14-Apr-2009): Base damage and scaling factor increased by 30%.
Patch 3.0.3 (04-Nov-2008): The Avenger’s Shield bounce distance has been reduced to 10 yards (from 15).
/Patch 3.0.2 (14-Oct-2008): Avenger’s Shield (Protection) cast time reduced to .5 seconds, duration increased to 10 seconds.
Patch 2.4.0 (25-Mar-2008): This spell will not 'jump' to targets that are under the effects of crowd control (such as sap or polymorph. There are some exceptions to this, such as fear, Avenger's Shield will still bounce to targets affected by fear.
Patch 2.1.0 (22-May-2007): Avenger’s Shield no longer has a minimum range. It may be used on any target within 30 yards.; The damage portion of this ability will now be applied even if the victim is immune to snare.
Patch 2.0.10 (2007-03-06): The slowing affect from "Avenger’s Shield" is now considered a snare, so snare removal and immunity affects will now work on it.
Patch 2.0.3 (09-Jan-2007): The bonus "Avenger’s Shield" receives from bonus spell damage effects has been increased.