"he made an incantation, he wore himself out"

In any given fantasy magical system, a person must have limits to his magical abilities, or the story has no conflict: the magic can overwhelm the other side. A theory of magic is that energy is taken from the magician/wizard to power the spell itself.  Depending on how strong or advanced the spell is compared to the caster, the spell can drain the magician’s energy and make him or her extremely weak.