Something physical at first- something low-scale, just enough to keep his attention on us.
Then, as soon as Scowl has ahold of it, we're going to bring out a keystone... or whatever it is that Delphinus has of his, if they agree. Just let it show for an instant, then box it up and pass it to one of several people in the crowd wearing cloaks, who will have been hidden from the start of the game.