Critical Hit said:Maximum Damage: Rather than roll damage, determine the maximum damage you can roll with your attack. This is your critical damage. (Attacks that don’t deal damage still don’t deal damage on a critical hit.)
Extra Damage: Magic weapons and implements, as well as high crit weapons, can increase the damage you deal when you score a critical hit. If this extra damage is a die roll, it’s not automatically maximum damage; you add the result of the roll.

(Dungeons & Dragons)
Rulebook featuring "high magic" options, including a host of new spells.