[@Zetsuko] Ah, no, that's a Yu Yu Hakusho reference to a similar concept. Lemons might have gotten it from somewhere different. Essentially, he's storing excess magic power in a valuable stone capable of containing magic energy over a period of 6 years, and he exhausted it on one strong spell.

EDIT: It exhausted our resources. It was not an easy victory, and even if it was, I find it preferable to an impossible fight.