Birth, and Death.

The planar schema of Spirit < Continuum < Void contributes a law of reality that is poorly understood, and barely grasped by the intelligent peoples of the world.
When a new life is born, it creates a tiny, delicate void in the fabric of the prime plane, a new space that did not exist previously. This tiny infinitesimal speck of absence is a singular point to which void connects to void, passing through the continuum of the soul-plane and the ephemeral echoes of the spirit-plane to connect to that newborn intelligence.
Void returns to void in time, but soul energy is drawn through that fading portal, where it passes through the spirit plane to become sheathed in the protective but impermanent energy and ultimately then settles in the living vessel that birthed it.
A soul will remain in the vessel for as long as the sheathe of spirit exists, and while it can be invigorated with other types of energy for a time, the spirit will always fade in time. Without the protective sheathe of spirit energy, the soul that has been hidden within the vessel is drawn back towards the continuum, passing again through the spirit-plane to create an imprint of the vessel as it was.
This imprint is usually created with high fidelity in regards to its thoughts, emotions, appearance and behaviour. Some factors, such as high levels of stress, the passage of time, extraplanar influences, and intentional intervention can affect the quality of the imprint, leaving unbalanced and sometimes violent shades of the being that once existed.