Defeating the Demon Lord's a Cinch If You've Got a Ringer Novel Vol.4 (US)

The hero's party is off to the water capital, Cloudburst, in order to contract with more powerful spirits. It's a big endeavor, and they may have bitten off more than they can chew, but not to worry--priest Ares Crown is not far behind, and this time he's got powerful mercenaries with him! One way or another, he'll keep the hero safe, even if it means following the party to the bottom of the ocean!