A Soul in Xevv is a type of Metaphysical Energy capable of storage. Generally, a Soul stores things such as memories, personality, and knowledge. They are always bound to a body until they die or become Immortal. All living beings, planets, stars and other celestial bodies have Souls, though most use them to store different (or no) information.
Origin
Souls were created by the God athena in the beginning of the second universe, after the first was naught but featureless clay and gas. They were created just to store unique information of every object in the universe, but ended up offering far more complexity than intended. As such, rules for them were quickly established (before the concept of death was created) by thoth.
Unlinking
Souls are mediated by the God Thoth, who sees that they pass from Reality to Flux properly. Thoth is incapable of error, having created this system, and so no error in the process has or ever will occur, as long as heaven still exists.
Thoth does not oversee Souls which have exited Reality abnormally (ie. Immortals.)
Death
Death is defined by the moment a Soul detaches from its body, which can happen for a multitude of reasons dependant mainly on the body. When a body dies, it releases a significant amount of Metaphysical Energy relative to the size and complexity of the body, as well as the complexity of the Soul's contents.
Binding Errata
On extremely rare occasions, the binding of a Soul to a body can erroneously extend to inanimate objects. This creates a Soul Gem which may be used to bind multiple souls to one body, a phenomenon only discovered in the Deceiver Universe.