Fire on the Water begins shortly after the end of Flight from the Dark. The Darklords' armies have laid siege to Holmgard so the king decides to send you - Lone Wolf - to Durenor to remind them of the treaty and get some help.
Almost immediately you're waylaid by traitors, and the ship you're on is damaged and then sunk by saboteurs halfway to your destination. Catching a coach, it's attacked by a Kraan. And one of your fellow passengers attempts to assassinate you. If you get through that and manage to navigate the bureaucracy at Port Bax, undead Helghasts, immune to normal weapons, dropped off by Kraan try to stop you in the mountains.
Assuming you get through, and haven't lost the Seal of Hammerdal, the Durenese mobilise for war and set sail for Holmgard. They also give you the Sommersword, a +8 sword of Undead-slaying. Which is fortunate as the Durenese fleet is attacked by a fleet of zombie-crewed Death-hulks, lead by the traitor wizard Vonatar. Getting through that, the Sommersword uses the power of the sun to destroy the Darklord Zagarna, after which the Draklord army flees in panic. Sommerland is saved!
This book has differences in style to Flight. There are more "pick a random number to find out what happens" and less "You can go this way through the woods, or you can go that way through the woods, but you may as well pick at random". Lone Wolf seems better equipped to deal with the countryside in Flight than the towns he finds himself in in Fire. Rather than the "regular" Giak forces of Flight, we meet a lot of saboteurs and agents and "special" undead forces. If I have a problem with it, it's that the undead Helgahst are introduced as unstoppable without a magic weapon, but then you very swiftly end up with the ultimate undead killing tool. And get to keep it for the rest of the series.
Anyway so much for Fire on the Water. My only problem is that Vonatar the traitor got away. Grr. If only there was an adventure to hunt him down.
 A black, leathery winged flying creature used by the Darklords for their airmobile attack, but also used in a close air support role. "Kraan!" goes up the cry when they fly past. "Kraan!" "Kraan!" "Nazgûl! Sorry, Kraan!"
 Technically one should call "Winged Nazgûl!" but presumably if they were riding horses one would be more likely to have time to add detail.
 Hint: It's the one who looks evil in the portraits.
 Vonatar! [Shakes fist]
 Regular in this case including the cold-blooded reptilian Gourgaz