Soul death

Soul death is the process by which a living entity becomes a true undead. A living soul bound within a dead body will experience this as it attempts to pass on to the river Styx the way it was meant to. Prior to soul death, an undead person can be restored to mortality by restoring their body to a liveable condition and breaking whatever bindings are holding their soul in place. However, once soul death occurs, the soul will be left in too low energy of a state to manage a proper living body, and thus will kill any body it inhabits. Proper revival is still possible, but the price is often too high to be worth paying.

Effects and Ramifications
An undead prior to soul death is much like a marionette on a string. The living soul is improperly fitted to the dead body, only remaining because of whatever binding holds it there. In soul death, however, it will enter a more malleable low energy state akin to a spirit more than a living being, and will directly animate the body rather than relying expressly on muscles and nerves.

Soul death causes incredible shock to an entity, leading to mental stability loss, lasting anywhere from a few weeks to indefinitely. This period is a very dangerous time for an undead and anyone or anything around them, as they will act completely unpredictably and thoughtlessly. These are the undead you see wandering the woods and sand wastes bludgeoning strangers until some Noble hero is said to come and lay waste on them.

If they survive, or return from this state they will become sentient, often far removed from who they were on a very deep level. Undead are often devoid of the morals, joys, or emotional needs and guidelines that drive living beings, making them fairly dangerous, though there are exceptions.

What is it though, really?
Soul death is a misnomer, as a result of how little is known about it, the belief is that the soul dies and loses all of its energy, leaving it unable to pass on at all.

Almost nobody knows this aside from a very small few. The truth of soul death is that the soul doesn't die, it flees, abandoning its current life's development. This goes into the nature of souls themselves as they grow over time and change depending on what happens to them in their lives. The river Styx washes off most of what they carry but certain important things. Strong beliefs or discoveries will often remain and effect them in future lives. A soul that "dies" essentially abandons all development from that life, and leaves the essence of the person that it was in that life behind in order to continue its natural cycle.

This means that although the number of undead may increase in an area, the number of available souls will not. The immortal souls carry on unhindered and unfettered, while the once mortal parts and the remains of now abandoned lives will wander until they are destroyed, and even then they may linger in Limbo for eternity if they please.

For more on soul death, and the nature of souls, see (Souls).