Second Law of Magic
Each living creature has a limited supply of magical power at any given time, called Imperium, limiting the quantity or strength of spells cast.
All creatures have an innate reservoir of arcane power to tap into when powering their magical abilities, their Imperium. Once used Imperium must be replenished, typically through rest and meditation. An individual cannot cast spells requiring more Imperium than they have available. Spells can be amplified either through the use of extra-magical materials or items such as wood from a Giftwood Tree, or by pooling arcane resources as a group to cast the same spell together. Runes use the caster's Imperium when created or refreshed, rather than when activated for use, making them an effective method for storing Imperium for later use. Runes also do not require the presence of their original scribe for use, making them the only way for the general untrained populace to access most spells. Unfortunately runes have not yet been discovered for all magical castings, nor are they flexible in their manner of interpreting the scribed spells, and so a mage is still required for casting in many cases.