The words carried the will of power, bringing forth into reality the true nature of the great magic cast by Sirius as the clouds parted, revealing a heavenly, yet horrifying light. It bore no resemblance to the sensation felt by mana, rather--this was true divinity which was invoked.
By the origin of this great magic, there is no evading it once the name of Susanoo is uttered. Once the name of the principal deity leaves the lips of the caster, the concept of divine wrath is ingrained into them--forcing only one option to the unfortunate soul on the receiving end of the spell.
This was the result of causality, a concept mortals must face when confronting the truly divine. Sora could do nothing besides watch in horror before the divine light filled everything, even his vision.
"--I warned you, buddy. I did my best to hold it back for you, but that's the price of challenging the strongest."