This is a little strange. The summoners tried to bind the demon king with magic, and it was reflected back upon them. The summoners don't know how to release their own bindings? They should know, and that info could be relayed to the demon king, so he could release them. If they don't know, is there not a summoner's guild, like there's a magician's guild? This feels like a weak plot device to artificially increase tension.