Crusader Strike is a core paladin ability. It is an instant weapon strike that grants a charge of holy power, can activate procs, and counts as a single-target melee attack for purposes of the paladin's active seal. This ability is a melee special ability, not an actual spell, and therefore it is not affected by [Silence] or spell school locking abilities (the others being [Divine Storm] and [Hammer of the Righteous]). Like other strikes, however, it does not work with any [Disarm] effects on. All damage dealt by the attack is physical.
Patch 4.0.1 (2010-10-12): Became a prime glyph. Now increases the critical strike chance of Crusader Strike by 5% (previously reduced the mana cost by 20%).
Patch 3.0.3 (2008-11-04): Changed from +20% damage for [Crusader Strike] against incapacitated or stunned targets to a 20% mana reduction.
From Patch 2.0 through Patch 3.3.5, Crusader Strike was a talent in the Retribution tree. It was an essential talent for Retribution DPS, giving them a second attack.
Cataclysm
This section concerns content related to Cataclysm.
In Cataclysm, Crusader Strike will be a core ability for all paladins, gained at level 1. Retribution will get a talent that causes haste effects to lower the cooldown.
Patch changes
Patch 4.0.1 (2010-10-12): Damage increased to 120% weapon damage. The cooldown is now 4.5 seconds. 1 Holy Power point is granted when your character is at least level 9.
Patch 3.2.0 (2009-08-04): Damage reduced to 75% weapon damage to match the new 4-second cooldown. Mana cost reduced to 5% of base mana.
Patch 3.0.2 (2008-10-14): No longer refreshes judgements.
Patch 2.3.2 (2008-01-08): Now causes 110% of weapon damage and no longer gains any bonus from spell damage.
Patch 2.3.0 (2007-11-13): Cooldown reduced from 10 to 6 seconds.
[Crusade]
[Rule of Law]
[Sanctity of Battle]
[Vindication]
[Wrath of the Lightbringer]
[Zealotry]
[Silence] or spell school locking abilities (the others being 
[Divine Storm] and 
[Hammer of the Righteous]). Like other strikes, however, it does not work with any 
[Disarm] effects on. All damage dealt by the attack is physical.
Patch 5.0.4 (2012-08-28): Removed. Existing glyphs converted to Glyph of Avenging Wrath. 
[Crusader Strike] against incapacitated or stunned targets to a 20% mana reduction. 
