I got a couple- 1)- Ghost of a prehistoric sea reptile or possibly extinct giant shark. Who said the undead had to be people?
an aquatic monster
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This is a Bakekujira. Also known as a Revenant Whale, a Nightwave, or a Sea Reaper.
Through a large part of modern history, whales and similar creatures have been hunted for one reason or another. In the times of Atlantis and Lemuria, Mosasaurs and colossal whale things were hunted for their fats, teeth, bones, and ambergris, and in more recent time whale fat was used for lanterns and ambegris for perfume, not to mention the applications both had for potions and other magics.
In these times colossal sea creatures were plentiful in the seas, and towns and villages the worlds over could reap wealth and sustenance from hunting a whale, with it's meat and oil lasting for quite some time. This became much more widespread when it was more commercialized, and therefore more of them were killed. Some however, were more restless than others....
Unsure if the Bakekujira is the spirit of a dead or vengeful whale or sea creature that has been hunted, or some spirit of protection, or perhaps the whale's grim reaper, or perhaps simply a terrifying and vile omen, it appears as a massive whale like skeletal creature. It's face has a somewhat human appearance, as if a specter of death merged with the bones of a whale to seek man's revenge. It's bones can be seen pristine beneath a shadowy exterior, like a three dimensional shadow of sorts, made of a vile, foul smelling shadowy gel, in the vague shape of a whale surrounding it. It's fore flippers are close to a human'd hands in shape, and it's eyes possess an eerie glow, as does the foggy mist it sprays from it's bllowhole, which leaves a thick fog settling across the sea.
When the seas were turbulent, the skies dark, or in the dead of night, a Bakekujira may emerge from the briny depths with raucous splashing and rowdy cawing as strange, ugly fish and eerie birds would flock around it as the creature emerged from the sea and moaned it's mournful call, sounding like the song of a whale and the screams of the damned.
When one would arise, those who saw it would be afflicted with a curse. This curse would bring famine, plague, and fire to everyone around them, and the curse would spread to others as well, leading to pandemics across the countryside of wildfires, loss of crops, or strange deep sea diseases. It will also mark them, and be able to follow them wherever they go. From then on they will occasionally hear the mournful song of the Bakekujira as they pass by bodies of water. They may see the shape of it's fin in a pond that it would never be able to fit into, or see it's eyes peering out from a puddle. No matter how small, they will be haunted by the creature, and maybe even attacked. Some legends claim of an old sailor who quit his job to work on land and warned everyone of the Bakekujira. He did so for years until one day he was found with his torso cut away from his legs as if bitten off, his legs laying next to a puddle outside of the pub. This does not only mark you for the whale however, as the strange fish and birds that swarm around it will also feel this curse.
When they appear in docks and on the bay, they will float to the top of the water, and a frenzy of the strange fish will begin killing all sea life in the area, while the birds will swoop at it and consume the fish or vomit a black pestilence onto the ground. These birds will swoop at people, trying to bite them, and they look like humans stretched into the shape of birds with no feathers to speak of and massive hand like wings. Their voices sound pained and their caw sounds like a man's voice. These vulture sized birds will land on the beach and tear apart the fish that they catch, leaving behind rot and disease, and they will flap and fly at people and pets, scratch and bite at their faces, leaving them disfigured and sickly afterward.
The fish take on a similar hideous countenance, With the faces of humans, creepy hand like fins and foot like tailfins. They bite and frenzy, attacking the fish of the bay and tearing them to shreds, gorging on their flesh and breeding with the rest, leaving the survivor's offspring to be deformed and foul tasting. They will swarm anyone in the water, biting with human teeth and inflicting the same sorts of diseases and infections one may contract from a human's bite. They make gasping, sickly sounds beneath the waves that rise up as a horrid chorus along with the singing of the whale. Those afflicted by the whale's curse may see these fish as well, catching one when fishing, tasting it or seeing it's face when eating fish, or seeing them wash up on shore or peering from one's bathtub. The Birds may appear and sit perched at a distance or swoop at you from nowhere. It's not known how to break the curse, however some witches or holy men are able to do so, though not as much now, as true witches and those with divine magic are uncommon on Earth, however there is a good chance to find such a person on Urth.
Sometimes these creatures will wash on shore and have similar results, only to vanish into the sea again later. Yet other times they come on shore and begin crawling with their hands as if trying to escape. When these beasts are attacked and slain, it seems that they scream and sink into the water as a vortex opens around them, pulling in everything close. Some think it drags you to hell, others to an ocean dimension of nightmares....
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I thought a Bakekujira would fit the bill here, and since ancient reptile was said I turned it into a mosasaur combined with a sperm whale in terms of design. Good ideas all around you two!