Hydrodaemons are spoilers. Personifications of death by drowning, these froglike monstrosities hide in the most foul and dangerous waterways in the multiverse—typically the very waterways the PCs have been told they must never, ever enter or even touch. And that’s the problem with hydrodaemons—they don’t even need to kill you. They just need to knock you unconscious or overboard long enough to let the vile rivers they call home—Styx, Lethe, and countless more—do the work for them.
The Grand Survey (known to less reverent wags as the Clanking Calamity) sends arbiter inevitables to the far corners of the multiverse, trying to catalog vital facts and take a census of the populations of each layer. That so few arbiters return from the realms of the daemons galls the kolyaruts so much that this Survey they are sending well-paid mortal escorts for protection.
To pay off a debt, some sellswords are hired as marines on the Bleak Barque, one of the only mortal ships daring enough to ply the waters of the Lower Planes. The first captain of the Bleak Barque made arrangements with the thanadaemon Deacons eons ago, but he formed no such relationship with the hydrodaemons and their attacks are a constant nuisance.
A legendary artist and showman among the already legendary glassblowers of Videnza, Maximilian Forsythe has just completed a wonder of wonders: a living museum he calls an “Aquarium.” The star attractions are a bound hydrodaemon and a captured “mermaid queen.” Maximilian is not a complete fool—the spells binding the daemon are top-notch, and he has promised the creature a share of the admissions. But Maximilian has not counted on two things. First, the merchant nobles of Videnza guard their prerogatives as jealously as all new money does, and they regard the Aquarium as just another private zoo, and therefore forbidden by law to commoners. They have sent saboteurs to see that the Aquarium’s opening is a disaster. Meanwhile, though the mermaid is no queen, she is a sorceress growing desperate to escape. Through the glass that separates them, the daemon has been tutoring her in the spells necessary to win her freedom and release him from his bonds.
—Pathfinder Bestiary 2 67