EDIT April 2018:
Check out my updated take on this alongside some neat ways to change up Tomb of Annihilation (again) here!
Sometimes, the sweet embrace of death is all but impossible for countless damned souls. They are forced to continue to walk a dark path. For them, they seek to regain an essence of life and restore their shattered minds, lest they be lost to an abyss of emptiness and madness. Such a curse can be from a variety of sources; an exploitation of a life source at a deific level, exposure to/using evil magics for too long, a hex brought upon by a powerful caster or even a destructive force bent on draining life from all reality.
The Curse of Undeath is a vile magical power that breaks the soul and leaves the body as little more than a husk. These drained husks share little with their once living counterparts. They are merely corpses controlled and warped by dread energy. Any remnants of mind left in these creatures are stretched to madness and driven to violence. While many of them retain enough knowledge to fight as though could in life, it is impossible for them to engage in rational dialogue let alone interact peacefully.
Author’s Note: Dark Souls III is out and I love it! With that, here’s an optional rule for handling a curse similar to the Undead of the Dark Souls universe. If you don’t want to literally be running Dark Souls, it shouldn’t be too hard to reflavor this into a different dread curse of sorts. If you’ve ever played Eternal Darkness: Sanity’s Requiem, Anthony’s chapter certainly comes to mind. One last disclaimer, these are mostly random ideas in my head, in an attempt to make a Dark Souls inspired mechanic. As such, it’s more than likely that this idea is a bit clunky. It will probably receive a revision in the future, if reader interest is there.
An Affliction of Body, Mind and Soul
As the last of life’s flames extinguish, you become less and less human. For every time you die, a piece of you is lost when you’re brought back. Only through restoring your humanity can you truly become whole. Even before the curse has truly taken hold of you, its dark power has tainted you indefinitely. You may currently look like one of the living, but heinous power seeks to transform you into one of many mindless and angry dead.
The source of the curse can be from one of several horrible events. Perhaps the Gods attempted to prolong the life of themselves and their followers by draining life essence from the planes. Perhaps an entire area was desecrated by overuse of necromancy. No matter the cause, the results will turn apocalyptic in the long run as people without souls begin to wither into deranged husks. Even before before truly becoming one of the Undead, the cursed already suffer greatly. Upon being afflicted by the Curse of Undeath, you automatically fail death saving throws when brought to 0 HP. You must still roll a Death save. Upon failure, you receive two failed saves instead. Getting a natural 1 causes three failed saves instead of two.
Image Source: Revenant – oevrlord
Based off of Dark Souls, which belongs to From Software. Made by Doctor Necrotic, for Doctor Necrotic Media.