Godspeed to Your Death
Decay is a fatal disease acquired after a subsequent bout of Corruption. It is fatal 100% of the time, and it occurs in about half the people who acquire Corruption. Why some people acquire Decay after Corruption and others do not is unknown; there are only theories. Magicians, witches, and humans can be affected by Decay.
Content warning: talk of body horror
Kaz flipped the page of the huge tome and landed on a page that absolutely horrified him. He had heard of Decay, of course, but seeing representations of its manifestation was...beyond frightening to him. There was no hope or comfort when it came to Decay; there was only pain, misery, and death. (In some cases, death seemed like it would be a welcome respite from the suffering.) The worst part was the uncertainty; why some Corruption sufferers acquired Decay and some didn't wasn't clear, and it didn't seem like there was a consensus on why this happened. It meant that once someone contracted Corruption, it was a waiting game for a few months: would they succumb to Decay? Or would they be one of the lucky ones, who simply suffered from Corruption alone? Kaz shuddered and quickly turned the page. Corruption was bad enough; Decay almost seemed overboard. Everything about it was appalling and repulsive; there was simply no mercy for those infected with Decay. Kaz closed his eyes and shook his head, almost like he was trying to shake out the images that now invaded his thoughts. Kaz could cause Corruption with a simple spell, but the idea of Decay was one of the most frightening things Kaz had ever experienced.
Transmission & Vectors
Decay is only caused by a reinfection of Corruption; it is impossible for someone to spontaneously contract Decay.
As mentioned, Decay is simply a reinfection of Corruption. About half of those infected with Corruption contract Decay, and why this happens is unknown. Some believe it's a random event, while others believe it has something to do with how someone contracted Corruption. Presently, there is no universal opinion on what causes Decay in some and not others.
The symptoms of Decay are brutal. Decay appeares as soon as a few days after Corruption to a few months; no known case has exceeded a year after acquiring Corruption. Besides the continuing symptoms of Corruption (high fever, bleeding out of various orifices, decreased organ function, skin sloughing, paranoia, hallucinations, and delirium), Decay is marked by excruciating necrosis: various organs, limbs, and other appendages will begin to painfully necrotize. Many with Decay end up with far less limbs, organs, and appendages than what they started with. In some cases, the affected can become either partially or fully paralyzed, with the only sign of life being the terror in the affected's eyes. The name of Decay is quite literal; people decay from the inside out, often in painful and gruesome ways.
The only treatment for Decay is comfort measures; there is no cure. Healer magicians can only attempt to treat the pain and necrosis, so perhaps someone can die somewhat peacefully.
The prognosis for Decay is extremely poor. Once symptoms are present, the affected can be sure of their impending death.
Anyone that has had @Corruption can be affected by Decay, regardless of age, gender, habits, etc. For most Thenians, this is what makes Decay so frightening; it easily kills those even in their prime of life.
Obviously, the clearest way to avoid Decay is to avoid contracting Corruption at all. If one does get Corruption, it is then unknown what causes certain cases to progress to Decay and not others.
Like Corruption, Decay cannot be spread from person-to-person. It can only be acquired after a bout of Corruption. The idea of a Decay pandemic (if it could be transferred from person to person) would be absolutely terrifying to any Thenian.
The first case of Decay was recorded far after the first case of Corruption (which has been in historical records as far back as historical records have been kept). The first official case of Decay was documented in the 3100s. This gap in recording leads researches to some questions: Is Decay simply newer than Corruption? Was Corruption fatal more often historically than it is now? Or was it a lack of knowledge that meant Decay was just considered a sort of "sub-Corruption" condition?
Decay is both cause for revulsion and sympathy from Thenians. While Decay is a sign of Corruption (which has its own cultural stigmas), if a person acquires Decay after acquiring a case of External Corruption (that is, someone used Corruption Magic on them), then that person is treated with the utmost sympathy and pity, as Decay is not a pleasant death. If Decay is acquired and its origin is Internal Corruption (i.e., their overuse of Dark Magic caused it), then the person is viewed as dirty and impure. They are often rejected from their friends and family and end up dying alone. An often muttered phrase around those inflicted is some variation of "may the gods bless you," since they are the only ones that can offer any help to those with Decay.
Well well well, here I am, back with another digusting and debilitating disease! I am fascinated by magic eating someone up, so Decay and Corruption are the result of that. I based much of my symptoms on that of rabies. (If you haven't read about rabies, do yourself a favor and don't. Horrifying.) Thanks for reading!