Initially an annoyance at worst with few fatalities, as this disease stole organelles from their hosts they had to find an easy way to escape, and thus began to target organelles which would allow them to produce digestive enzymes—they began to eat their hosts from the inside out. The result was that the hosts were left with open sores on their bodies, from which the disease was released. It goes on to enter new hosts through their gills or open wounds, disproportionately affecting cish because of the easy access granted by the filter baits in their gills.
With no immune system to protect the cish, the cish plague swiftly spread throughout the world, infecting and killing nearly every cish in a given region in only a matter of weeks. The sheer amounts of death coated the seafloor with carcasses, which were a feast to scavengers and decomposers. Due to horizontal gene transfer from the hosts of the plague’s ancestors allowing them to permanently produce luciferin, the fresh piles of carcasses appear to be surrounded by a faintly glowing purple mist. Only one cish, the Snoodcish, managed to just barely escape extinction—the plague swept through its range during its breeding season, and the disease could not tolerate freshwater due to osmosis, so sick individuals did not carry the plague to the freshwater breeding grounds.
In the end, all cish went extinct except for the Snoodcish. Though the plague disproportionately affected cish due to the nature of their gills, it also wiped out the Carpotesta Devoratori and the Protodevoratori, which were already under heavy competition from other Geletaventrians. Without any reliable hosts nor a way to go dormant until the Snoodcish returned, the Cish Plague, too, died out.