Caster's Noble Phantasm used to sever magical contracts and spells.
The only Noble Phantasm wielded by Caster throughout the Holy Grail War, Rule Breaker is a jagged dagger that possesses the ability to sever enchantments, magical contracts and other spells cast by magi. While worthless as a weapon, it still proves a threat as its magical properties allow it to sever the bonds between Masters and Servants, thus allowing Caster to assert control of a Servant through obtaining the Command Spells of a Servant's original Master.
Caster would utilize Rule Breaker to gain control over Saber and Archer during the Unlimited Blade Works route of Fate/Stay Night. For Heaven's Feel, Shiro would create a projection of the dagger in order to free Sakura from the influence of the corrupted Holy Grail.