"Oh? Not going to attack this time? Very well, then--"

The mage knows that Katsuki is waiting to counter whatever he conjures. If the boy can manipulate explosive power then he wonders how well it will fare against some less...tangible elements.

His shadow seems to waver. Then like a drop of ink on the thinnest of paper, it blooms: spreading from his feet across the ground towards Katsuki, before a great maw explodes from the ground and tries to swallow the boy whole.