There are two magic stats in the game: Magic Affinity and Magic Points (or MP). Magic Points are used to cast spells and every spell cast has a chance to cost Magic Points. Once a character has 0 MP, he/she can no longer cast spells. MP is completely regenerated after a night’s rest.
Each spell has an MP cost attached to it. Every time a spell is cast, the caster may roll a Magic Affinity roll. Subtract the number of successes from the Magic Affinity roll from the MP cost of the spell cast. The resulting number is the amount of MP the caster lost from casting the spell.
Example: A spell caster with Magic Affinity of 20 casts a spell that costs 5 MP. The caster rolls the Magic Affinity roll (in this case, 2d12) and gets 2 successes. This means that the spell cost 3 MP to cast (5 – 2 = 3).
It is possible for a spell to be cast that costs no MP at all if the Magic Affinity roll is high enough.