The Spheres of Magic
The Spheres of Magic are energy centers resting within the body, along the spinal column. When attuned to a Sphere, the mage can manipulate its corresponding element in the outer world. Most mages are only attuned to one or two Spheres in their lifetime.
But the powers of the Spheres are not endless: prolonged use can drain and weaken the user. Or, if tapped by a necromancer, can turn the host into something entirely inhuman.