I think Mathias was expecting more from his father, but it was easy to see just how outclassed the stripped down drach was in the face of his son's magic. How had the young man gone so long under the thumb of one shackled by the loss of the majority of his strength? The fallen drach couldn't even swell his size, his massive body all physical, not augmented by magic.
And the subtle fizzle of what remained of his rainbow energy made me sad. I almost looked away, but for the moment of elation turned to horror on Mathias's face as he, too, realized just what he'd been afraid of.