(artwork by Sourine)
Nobody really knows when Grey Plague appeared for a first time. Certainly it has been hundreds (if not thousands) of years ago, perhaps even in the times of the Great War. It is not known whether it was created as a biological weapon of the Old Ones or its origin is quite different.
One thing is certain - the plague changes people into monsters. Spores of the disease attacks every cell of the the host's body, leading to his death. Despite the apparent demise, disease transforms the victim's body, sustaining his life in a unknown way. Thus, victims of the plague - often called the Dead Ones - practically does not need to eat of drink (though if it does not take the "replacements" for their diseased tissues - especially if they are injured or otherwise damaged, eventually they will begin to rot and decay), also they are resistant to the effects of aging (finally they are dead - at least in some sense). Unfortunately the course of infection is horrible and extremely painful, which results of falling the victim of the Grey Plague falling to madness.
As the outbreaks of plague have not appeared since ages and infected with the disease can release spores only once in a hundred years, the amount of Dead Ones is getting smaller. Moreover, they almost did not appear on the surface of Ortix - especially that they are treated there as a "ordinary" undead. Most of Dead Ones hide themselves deep in the Tomb of the Exiled.
Dead One as a character class
(click to enlarge)
Additionally, GM can add some extra rules, such as doubled (or even tripled from some experience level) HP regeneration rate, reduced need to eat and drink, vulnerability to Turn Undead power etc.