[Well, he knows Chaos is there; he just doesn't know what Chaos is capable of. Other than royally pissing off his Mother, that is.

Sephiroth twists to the side with the aid of a quick wingbeat as the fireball erupts towards him. The fire licks his stomach but only does superficial damage. He ignores it as a materia lights on his bangle.]

So the beast displays its nature at last. Thundaga!