After the War of Titans, the Mortal Soul was created by the Wildgod Zirche to give life and sentience to certain animals, which in turn gave rise to the Evolved Races, who would eventually become what we refer to as the common, native races of Nym. Exactly why Zirche created the Mortal Soul isn't known, but likely it was a wish to create new life that could populate a world that had been ravaged by the Elemental Titans, and in the process heal it. To create The Mortal Soul, Zirche used living energies of the world, and also infused her own soul into it to empower it. Overall the Mortal Soul consisted of four main essences that are the core elements that build up a mortal's body and soul:
The Flame of Life; which sustains the Mortal Soul and gives life a cage to contain the soul. In time the flame will weaken, until it is finally extinguished and the soul can leave; giving life its mortality. Zirche used her own, flaming soul for the process.
The Pool of Mind; which gives the Mortal Soul a connection to the world's energies, and created a basin for mana for mortal beings, as well as the capacity for intellect and wisdom. Zirche obtained this essence from the magically strong Worldsea.
The Body of Flesh gave the Mortal Soul a shape and vessel, and with it mortal life is given a physical form; with blood as its source of power. A fallen Earthen God was used by Zirche to contain this essence, though who he was exactly is unknown.
Finally, the Gust of Spirit, which gave the Mortal Soul purpose. Mortal life could now gain sentience, learn and feel the urge for existence. Most of all they were free to grow, evolve and act independently. This essence was willingly given to Zirche from the Kal'Mar, and completes the ability for Aetherial to merge with a mortal's soul, and become a Mortal Spirit.
After
the Mortal Soul had been assembled and used to elevate the first
Evolved Races, Zirche would later (as a result of the War of Eternity) merge the Mortal Soul with the Worldsoul, and
made all life in the world mortal. This meant that no living creature
could ever escape death, and even beings that live forever could be
vulnerable. This included all the gods and spirits that reside on the
planet Nym.
It is speculated that Zirche did this because it was the price needed to ensure mortal life could continue to thrive after the War of Eternity; by making all life in Nym mortal. Or it was a measure she made to protect the world from the Creators, to make sure that another War of Eternity could never occur again, since the Creators could now be killed on Nym.