Mana, Blood, and Psi
All spell-casting classes (and those with spell-like abilities) have innate sources of power to draw on. For most, this inner magical power is called mana. All spell-casters draw on a finite pool of mana. After this is depleted, they must rest to recover mana points. For demons and vampires, their magic is derived from the blood of others. Unless they feed to replenish their blood power, they will suffer and die. For psionicists, the mental power they generate is known as psi. Like mana and blood, psi is a finite resource. It can be regenerated with rest. Even the non-magic classes have innate mana within them, though they have neither the art nor skill to release it.