Transmission & Vectors
Can be consumed during the larval stage, once mature the parasite can enter any orifice of the body. When inside of a non-sapient creature the parasite propagates within the host, swelling the body as the thousands of larva grow into adults. Once critical mass is acheived, the oldest parasite drives the host to rampage around sapient species or to burst itself inside of a potable water source.
Creation of Old Tsae blood science, designed to bolster ranks of the undead horde by taking control of living defenders.
Very distinctive black patches of necrotic skin appear on the body within ~12 hours. An hour after this the parasite takes control of the host. Before this there are no symptoms.
Saltpeter burns the parasite, even a small amount with destroy the parasite. However this is only usable to "save" the host before the parasite takes control, afterwards the only treatment is execution.
It is almost impossible to know if one has been infected until the patches of necrotic skin develop. By then there is only an hour to administer saltpeter to kill the parasite before it takes control of the host.
Survivors of the parasite can look forward to possible permanent nerve damage within the extremities, a massive increase in kinesthesia, and a permanent loss of facial muscle control resulting in slack dead expressions and a major difficulty communicating vocally.
Hosts & Carriers
Any living creature except for Tsae, and carriers of Tsae blood(I.E. Half-Breeds) can become carriers of the parasite.
Regular consumption of the only preventative (saltpeter) is harmful to the bodies of most beings as it causes kidney damage and anemia. Magical detection used on consumables also works but is widely unavailable to most.
Random occurences of the parasite are mercifully rare, as it is created using Old Tsae blood science. As such outbreaks are usually caused by them or by agents acting in their stead. This is not to say that it doesn't exist in the wild but its environmental needs are fairly scarce on the main continent.
The first recorded instance of the Hatraxxian Parasites was during the Siege of Lamelorn during the Great Undead Invasion. The entire Elven city was turned into a morass of infected within a week. Independent investigation after the fact by the Elvish Authority determined that a horse had been infested and made its way into the city's reservoir, contaminating it utterly.
Infected are rounded up and killed. Most see it as a mercy killing.