Codex: Mana

Mana can be categorized into two distinct categories. Light and Dark. They are fundamentally incompatible with one another. You are either with the Light or with the Dark. Mana is stored in a caster's extremities, it can actually be absorbed through physical sucking. So a weaker mage can leech off of a stronger mage to improve. In mortals, only a set amount of mana can be used per day without harm. Overuse starts with exhaustion and ends with internal bleeding if pushed, one should never cast from HP.