Ritual
Rituals are invocations of the deepest and oldest magic. The kind used to forge the world. The kind used in the distant past to level mountains and fell empires. What powers the effects of a ritual? Is it the Twelve Immortals? Is it an even higher power? No one can say for sure, but it seems to be written into the very fabric of the world itself.
Rituals are nowhere near as prevalent as spells — they're complex, powerful, and dangerous. A mage may spend the majority of her life creating a ritual. To learn a ritual might be the very goal of a role-playing campaign. A ritual might end a drought, destroy an ancient evil, or even grant immortality.
Rituals are special in that the participants may not even know they're performing one! Imagine a farming village full of bright faces and hard workers, who gather on the vernal equinox to pray for healthy crops. They make offerings of what remains of last year's harvest onto a pyre. They slaughter a bull and offer its blood to the heavens. Their voices unite in ceremonial litany. Little did they know that they just successfully performed a minor ritual to bless their land and nurture what grows there.
Rituals are a petition to the laws of the universe. Rituals are not something that you can just fire off during a round of combat. Spells require fuel, but rituals require offerings.