The Red Plague was a plague that swept through the city-state of Vesuvia, and much of the world, killing many people. Its name comes from the signature symptom of the sclera of the eyes turning red.
Not many symptoms are known. One of the telltale signs are the sclera of the eyes turning a bright red. In later stages, the area surrounding the eyes also becomes a sickly red shade. Bright red veins appear around the eyes, making it appear as if the eyes are weeping blood. These veins are also prevalent at the extremities of the body. In a flashback during Asra's route, Lucio, who had the plague at the time, has a nasty cough and appears sweaty and gaunt.
Most of the people who catch the plague die in a matter of days or weeks. Lucio was able to survive for months indefinitely before he was 'murdered'. This is thought to be because Lucio himself was the source of the plague.
Lucio unwittingly started the plague when he made a deal with the wyrm of pestilence. Intending to kill his parents, Lucio offered their hearts to the wyrm in exchange for a "foul disease" that he could use to weaken them. The wyrm imbued Lucio with the sickness, which he brought back to his tribe. Both of his parents quickly fell ill, and Lucio killed his weakened father with ease. His mother, however, was able to battle the disease, referring to it simply as a "summer cold." Lucio, terrified of his mother's wrath, ended up fleeing his tribe. After joining a mercenary band and traveling around with them, Lucio began to spread the plague with him.
At a certain point, the plague manifested in the form of beetles. The beetle's bright red color became a popular dye, which helped further spread the plague.
After Lucio settled in Vesuvia and eventually became its count, the Red Plague hit the city. Lucio finally succumbed to the sickness he carried, causing the palace to put out a call for anyone who might be able to help cure the plague, offering resources and space to work. When the plague began to worsen, Asra and the apprentice discussed what to do. The apprentice wanted to stay and help find a cure, but Asra wanted to leave Vesuvia to escape it. After an argument, Asra left Vesuvia alone. The apprentice became the assistant of Dr. Devorak in hopes of finding a cure. However, the apprentice eventually succumbed to the illness. They were shipped to the Lazaret to be cremated, and burned to death alongside other plague victims.
When Asra returned to Vesuvia and discovered that the apprentice had perished, he was devastated. He began a plan to bring them back, which involved him working at the palace under the guise of assisting the effort to find a cure. There, he ended up working alongside Julian. Julian eventually discovered the cure to the plague after contracting it himself. After speaking to the hanged man in his realm, Julian was told Lucio was the source. Before Julian could act on this knowledge, the count was burned to death in his bedroom. The 'death' of Lucio led to the end of the plague. In the aftermath of the plague, Julian would begin wearing an eyepatch to hide his plague eye, a sign he has gotten the plague and been cured.