The Plague

Origin

Highly contagious, mysterious origin. Secretly known to be a direct product of the Moon Presence himself.

Effect

Plague propped up overnight with the centerpoint seeming to be Lothric. Lead to the destabilization of the Kingdom of Lothric and Alusfarian politics as a whole. For some reason, the Quarlanian Kingdom has been better able to manage the outbreak. Lead to Lothric being shut down and a quarantine being placed. Eventually lifted and relaxed by Azazel when he rose into power.

Symptoms

Anywhere from 6 months to a year incubation period from infection to death. Symptoms include black sludge pouring from orifices, bouts of delirium and madness and random mutation towards the end of the incubation period ranging from mild rashes to the development of new smaller limbs. In earlier stages, treatment can be made with the use of the lesser restoration spells and similar healing magic to delay the onset of more severe symptoms. Later stages, especially well into mutation, restoration spells only prolong the inevitable and hardly have any sort of effect.
In more advanced cases, the infected have been known to completely lost all sense of sanity and revert to a more primal and savage nature. Mutations are more severe and the black sludge will be a much more common occurrence. 

Transmission

The Plague transmits itself through inhalation of its spores that take host and manifest itself in the people's bodies. The spores, once they take root in the host, spread throughout the body ultimately result in the symptoms and mutations. The black sludge expelled from the infected's bodies is a hotbed of these spores and after prolonged exposure to the sludge without the proper mask filtration any persons near these hotbeds suffer a high risk of infection.

Plans to Harness as a Bioweapon

While Azazel is prepping the resurrection of the Moon Presence, he is using the Kingdom of Lothric's resources to create multiple, more advanced strands of the Plague. One used as a virulent bioweapon that greatly enhances the efficacy of the transmission and advancement of the sickness, and a different strand that harnesses its power of mutation to create monstrosities to use as weapons of mass destruction. 
In either case, it appears the Kingdom under Azazel's guidance has been able to control the minds of some of the deeply infected using an unknown spell housed in mysterious dark purple looking gems. The wielder of said gems can command the affected to do their bidding. Once the crystal is destroyed, the influence over the affected fades.

Cure

Known: Caspian's Flail - the only item known that extracts the plague and reverses its effects
Dragon's blood transfusions have been secretly known in some circles (the Draconic Monks) to make you resistant to all disease, but is especially effective against the Plague.
Demons and some of those who have partaken in demonic flesh have also been found to be resistant to the plague, but these victims ultimately succumb to other illnesses and horrible mutilation in the process as a part of the ritualistic consumption.

Type

Magical