Hardly anyone gave the whale-oil works a second thought until the deaths began. Neighbors reported a horrible shrieking howl and found a room soaked in gore. Urchins saw a huge phosphorescent shape glide through the fog. Something huge and horrible stalks the streets of Endon, seemingly killing at random.
Pigeons are not very bright. Their feeble little souls rarely survive being polymorphed into whales. But once, by accident, one of Bartelby assistants polymorphed a stray dog. Dogs, as any professional wizard knows, have the greatest soul-to-mass ratio of any mammal. No sensible wizard would credit them as distant evolutionary relatives to whales, but it's true. Being a whale was, for the dog, a deeply confusing experience, so after death the dog-whale stuck around to see what the fuss was about.
Endon is haunted by a cetacean ghost. It glides through the streets on foggy nights, a translucent outline paddling through clouds, buildings, and cobbled streets. It is a lonely whale. Sometimes, it howls.
Ghost WhaleHD: 7
Wants: friends, belly rubs, krill.
Armor: none. Requires magic weapons to hit. All the usual ghost immunities (charm, sleep, cold, electricity, etc.).
Move: 2x fly, swims through solid objects
Damage: 1d8 life-draining bite, whale howl (see below).
The ghost whale's howl affects anything in a 50' cone in front of it. It starts as a low rumbling, then rises to a painful, oppressive shriek that bursts blood vessels, cracks skulls, and pulps eyeballs. It takes two rounds to reach full power. In the first round, anyone in the area of effect can try to flee the area. If they remain inside, they take 1 damage. In the second round, anyone in the area must Save or die. Typically the ghost whale only howls once a night.
The ghost whale is also a dog. It wasn't a particularly good dog in life but it wasn't vicious or spiteful either. It wants to chase carriages, sniff garbage, and fall asleep in front of the fire; all things made difficult by its spectral existence. It might be possible to tame it.
Treasure: 1d10lbs of ambergris ectoplasm (ghost whale barf). Like half-real solidified glowing air. Worth 1,000gp per lb to any wizard or perfumer in Endon. One of the few scents ghosts find pleasant.
Yes, it's just a reskinned Banshee.
So What About The Law of Conservation of Mass
Utter nonsense. Ever seen a fire? Reduces heavy wood to light ash. Burns oil and spirits into nothingness. Or consider ice. Allow a sealed container full of ice to melt and less water than ice will be found inside. No, I'm afraid mass is not conserved. It comes and it goes as it pleases.