The Bastard's Plague

In approx 720 ABM a disease known as Reginald’s plague spread across Enther over a 20-year span, wiping out 25% of the population. The common folk believed this was punishment from the gods for allowing a product of incest inheriting the crown. The disease killed indiscriminately, and immediate quarantine was given to those who became infected, with eventually people showing symptoms to be killed in order to prevent mass spreading. Warlocks, Scholars and Alchemists managed to create a vaccination for the disease and the disease was got under control after 20 years. Many of the royals, Princess Mathilda, Princess Eleanor, his son Prince Jon and other members of the court all perished from the disease. 
The only surviving heir was a nephew of King Reginald, Prince Duncan who would become the youngest King ever at that time at only 15. King Duncan would reign for 60 years and was known as the King of the People, using his power to truly rule and help the people of Enther. King Duncan negotiated trading treaties with the Kingdoms of the Entherlands and overseas, including the Tang Empire.

Transmission & Vectors

An airborne plague, the Bastard's plague was easily transmittable through coughing, sneezing and any other bodily functions that would result in a transmission.

Causes

The origin of the disease is unknown, however the common folk of the kingdom put this down to the rule of the King Reginald, The Bastard King. Born out of supposed incest between King Anders and one of octuplet sisters, Reginald's claim to the thrown was also debated by both the nobles and the common folk. 
It is said the disease, was punishment to the Kingdom for allowing such an abomination to take the crown. This was mentality was further enforced when after King Reginald's death the heriship passed to his nephew Duncan, who is considered one of the greatest kings in the Kingdom's history.

Prevention

Prevention for the spreading of the disease became quarantine and extermination. After attempting to quarantine the plague, King Reginald ordered the execution of anyone who displayed symptoms, though this found himself in a prickly situation when he and his immidiate family all contracted it, resulting in him ordering the death or his concubine and his children, followed by his own suicide shortly after.

Origin

Natural

Rarity

Unique