Shadow Magic

The shadow magic spell allows the illusionist to cast a quasi-real magic-user spell. 

This spell can be magic missile, fireball, lightning bolt, or cone of cold and will have normal effects upon creatures in the area of effect if they fail to make their saving throws. 

If saving throws are made, the shadow magic spell will inflict but 1 hit point of damage per level of experience of the illusionist casting it, regardless of which quasi-real spell was cast.

The caster is actually tapping a power source, even though the majority of the spell is illusion, thus the 1 hit point of damage per level of the caster.