The Plague Chalice

If the bearer of the Plague Chalice rolls any natural double when making a Casting roll (not including rolls of a natural double 1), the spell is cast as a perfect invocation, regardless of its casting value or of any other modifiers. However, each time the bearer casts a perfect invocation they suffer a Strength 4 hit with no armour save permitted (Ward and Regeneration saves can be attempted as normal).