The Lancet Fluke Seal spends the first ten years of it’s life as a bottom feeder like a ray, depending on stealth and the fact that it has a nasty taste to avoid being eaten. There are creatures that like the taste however, and many Lancet Fluke Seals end their lives young.
After a decade the survivors find an empty cave or a place under a rock and spin a cocoon around themselves that quickly hardens. The cocoon is poisonous to anything that might decide to eat it, killing them quickly, and can be dried by humans and made into one of the most deadly poisons in Acqua.
After a month the Lancent Fluke Seal breaks out of the cocoon and it looks very much like a normal fur seal except for a small lump on the top of it’s head. The seal then heads out and looks for it’s prey, great white sharks. When the seal and the shark meet the shark thinks the seal is prey and goes for it. The seal ducks under the jaws at the last second, attaches itself like a remora fish underneath the shark’s head with hidden suckers and sends a pointed spine into the shark’s brain within seconds.
The shark dies on the spot but the Lancet Fluke Seal now links it’s blood supply and nerves to the dead shark and takes it over, working the dead muscles and jaws and hunting with the shark, and then using the food that the shark eats for itself.
After a few days the shark begins to rot and brings all manner of creatures that see it as food from small fish to krakens. The seal plays dead to lure them in, strikes and eats everything it can catch, then swims to another part of the ocean and repeats the process. In the long term either the shark’s muscles will rot totally and tear apart, or something too big for it will decide to eat it. In either case the Lancet Seal quickly disengages, flees as fast as it can, leaving the body to be eaten or sink to the depths of the abyssal plain, and finds another shark to kill and take over.
Lancent Fluke Seals actively see humans and anything small enough for a shark to eat as food and will attack without hesitation, and the body they are controlling has often rotted to the point that it is mottled, stinking, and disgusting to look upon.They rarely possess anything other then sharks, as whales are too big, most krakens would tear the seals with their tentacles before they can get close enough, and human bodies are rejected as being too clumsy to catch anything in the sea with their hands and teeth.