Possession of the Mind

The ship wrecked off the coast of Taina was attacked by sea elves and mermaids that have been possessed by demons, who are in turn controlled by wizards.  They are hiding out in an underwater cave near the crash site and are trying to gain treasure and power so they can take over the nearby population and rule.  As the GM you can add the number of wizards, demons and elves as needed to make sure it's a challenge for your party.  The cave entrance is underwater, but the cave is good for air breathers.