The Soul is a central concept in End War Online, being the driving force behind life itself.
Every animate creature after the arrival of the Four Lords possesses a Soul. The Soul is necessary for life to exist, and is the manifestation of the force a being exerts on the world.
The Soul is where a being's Anima supply comes from, which allows one to perform various kinds of soulcraft. The quality of the Soul directly correlates to how much Anima one possesses, as well as the kinds of soulcraft that can be performed.
The True Soul is the original Soul, from which all other Souls were born. It was found by Lord Vallheim, but was created through unknown means long before The Four Lords set foot on Eden. The True Soul is considered the most powerful Soul, as it represents the collective force of all beings in Creation. The True Soul was used by the Four Lords to create the Light and Dark Souls.
The Lord Souls are the Souls possessed by the Four Lords. These four Souls were created within the Four Lords when they arrived on Eden for the first time, manifesting within themselves to be able to bring life to the world. They are extremely powerful souls, being only surpassed in power by the True Soul.
The Dark Soul is the powerful Soul possessed by Humans, created by the Original Lord Orcir. It is known to be one of the most powerful and dangerous souls, as it is capable of both creation and destruction, being an unknown and unpredictable force that is the manifestation of Human potential. It is forever intertwined with the Light Soul.
The Light Soul is the Soul possessed by both Angels and Demons, created by the Original Lords Loratiel and Iraam. It holds equal power to the Dark Soul possessed by Humanity, though it does not share the unpredictable and dangerous nature that the Dark Soul does. It is forever intertwined with the Dark Soul.
The Gray Soul is simply the Light Soul and Dark Soul fused together, and is held by Hybrids. It is suspected that this Soul was born naturally within Hybrids from Angels and Demons mating with Humans.
It is unknown exactly what kind of soul the Nephilim possess. Though with the abilities known to Nephilim, being able to harness the powers of all worlds, leads to suggest that they are in some way closely connected to the True Soul, perhaps just as much as the Lord Soul. As there are only three currently known Nephilm, it is difficult determine the true properties of a Nephilim's soul.
Void Souls are by far one of the most powerful and destructive Soul types, despite not being a Soul at all. Void Souls are originally normal Souls, but have been infected by the powers of Void to the point where it consumes the entire Soul, becoming dark and thick with Void, the original Soul being forever lost. The first Void Avatar is the most well known wielder of a Void Soul, when he gave in to the powers of nothingness and allowed his Persona Erebus to take control. A being with a Void Soul is considered, for all intents and purposes, dead, as a Void Soul is empty and dark, containing within it no life.
The Souls of the extinct Rythulian race were a fusion of the Angel and Demon aspects of the Light Soul. Created artificially in an attempt to create Nephilim, as the theory was that Nephilim were a direct combination of Angel and Demon. Though not Nephilim, the Rythulians did possess remarkably powerful Souls that could rival them.
The Persona of a Nephilim is unique in that it is an entity without its own Soul. Contrary to the belief that a Persona is a separate being of its own, Personas are simply entities created from a Nephilim's Soul branching off and manifesting into the Persona, becoming an extension of the Nephilim's Soul. This means that a Persona is permanently bound to the Nephilim, and explains how a Nephilim invokes and revokes their Persona.
And exception to this is Erebus. When the First Void Avatar gained a Void Soul, Erebus became a separate entity in itself, becoming self-aware and using its powers on its own accord.