Empire of Souls
Curse of Undeath
A mystical curse that plagues the whole world. It is not transmitted by a disease or a spell, rather it seems to strike semi-randomly: once a single individual in a settlement is cursed, it is likely more will follow soon.
Those cursed have their souls permanently etched into their bodies and no force can remove the soul. In the event of death, the soul begins repairing the damaged biomass with energy in the body. If the death was sufficiently “thorough” (complete vaporization, massive cellular damage etc.) the soul starts drawing energy from the Shadowfell. Depending on the injuries, the regeneration process can take anything from a day, to months. Unusual circumstances, such as proximity to Shadowfell, might considerably reduce the timeframe.
If something tries to impede the regeneration process, for instance chucking the body into an active volcano, the soul will start to gather even more power from Shadowfell at an increasing speed, until eventually the Undead turns into an Abomination of Shadowfell.
If the Undead loses its mind or dies enough times, they will degenerate into mindless Undead. They become mere husks who endlessly repeat the same actions they did in life. Before that happens, they turn increasingly feral, willing to kill and devour other creatures just to gain more energy. And even before that, it is common for Undead to start losing their memories.
Furthermore, all living beings sense that something is “wrong” with the Undead and any magical sensors detect them as Chaotic Evil, regardless of actual alignment.