If a natural double 6 is rolled when making a Casting roll, the spell is cast regardless of its casting value or of any modifiers that may apply to the Casting roll. A perfect invocation cannot be dispelled.
Source: Warhammer: The Old World Online Rules Index
If a natural double 6 is rolled when making a Casting roll, the spell is cast regardless of its casting value or of any modifiers that may apply to the Casting roll. A perfect invocation cannot be dispelled.