Critical Strike increases the chance that the item critically hits its opponent (normally it's 10% of hits are criticals). The increase in chance depends on the attack skill of the wielder. Add Crippling Blow to a loot weapon using Magic Item Tinkering to apply a bag of Salvaged Black Opal.
Formulas:
Melee Weapon Chance of Critical = (Weapon Skill - 100)/600
Missile Weapon Critical Multipler = (Weapon Skill - 60)/600
For example, with a base Sword skill of 400, 50% of hits would be criticals. And with a base Bow Skill of 360, 50% of hits would be criticals.