Soul

Soul is the term to represent the incorporeal essence of every inhabitant be it alive or dead in the New World. It would also seem to act as fuel for the casting of both Resurrection Magic and Wild Magic.

YGGDRASIL Lore
It was said that any person who got killed by the Death Knight’s blade would turn into an undead servant. What became of their souls were in fact, slaves to its master for the rest of eternity.

Additionally, there were Soul Eaters who consumed the souls of its foes in order to grow stronger. Ainz Ooal Gown is currently testing whether or not it is possible for Soul Eaters to remain permanently in the New World if they were to devour a soul.

New World Lore
All living and some undead creatures are known to possess a soul in their own body respectively. What represents the souls are believed to be the "minds" of individuals. There was even a job-changing item known as "The Soul of Cainabel's Blood" which allows a person to acquire the Blood Soul class.

Although people get themselves killed due to incidents or events of various wars between countries, their souls don't simply vanish after dying. Instead, it will leave the bodies, continuing to linger on in the location where they have ended up dying at. Hence, priests are often sent out to scout areas and use their spells for ceremonial prayers to cleanse their surroundings of undead. They have done so to purify the souls lingering in any area influenced by negative energy. If the negative energies are proven to be large-scale, massive prayers will not be enough to rid all of it away.

According to the priests' religious viewpoint, souls resided within the body of every living being in the New World. When any one of these bodies begins to rot, that soul would be summoned away by the will of the deities. They believed that the undead entities are responsible for the corruption done to the souls of the dead.

Meanwhile, Demons have the capacity to grant any wish in return for any person’s souls while the equivalent exchange is made between both sides. For starters, a demon could transformed into beautiful women to claim souls as part of that individual's deal.

Mechanics
As the living and specific undead creatures have a mind of their own intelligently, so do the souls which are bounded to each one of their bodies before death. It could be said that instant death magic lies in the Departed Souls System of Magic. However, the author Maruyama noted how there were other magic systems that hold instant death effects. For example, the Spiritual system magic also have it apart from that one magic system involving departed souls. When a person like Neia dies, their soul departed from the body while it gets swallow up by a world of invisible darkness they cannot see beyond the afterlife. In that kind of world is only the fallen ones' minds which they tend to reside and think without self-identity or awareness while immobilized. If a departed soul was as strong as Zaryusu Shasha, one should be able to perform spiritual actions without being immobilized by going out of their way to reject resurrection from the living. Nonetheless, any one of the souls can be brought back to the living as long as its physical body or corpse is present for someone to use Resurrection Magic on. However, third-party magics like [True Death], a 9th tier spell can invalidate such revival process from happening between body and soul in connecting together.

Along the way, the True Dragon Lords are entirely dependent on using the departed souls from the world of darkness whenever it comes down to using their unique Wild Magic. On the other hand, one could even use an unknown spell to gather up all souls of the living from within their surrounding or range of effect to prep their Wild Magic for long-term use. The Dragon Lord's Wild Magic is fueled by these collective souls. Ranging from lowest-ranking spells to highest-ranking spells of the Wild Magic System, each spell are varied by the amount of souls needed to utilize it. For instance, the more powerful a Wild Magic spell is, the likelier the cost of many souls that have to be used for casting it while as the weaker kinds require less souls to do so. As a matter of fact, once they are left with one soul remaining which is their own, a user of Wild Magic is capable of consuming it in order to cast a Wild Magic spell. In the case of Elder Coffin Dragon Lord, the souls he stockpiled and then consumed for the casting of his Wild Magic spells cannot ever be used again. When such souls are extinguished by the said Dragon Lord, it means that they are forever removed from the New World.

In the event that Cure Elim Los Malvar was to use up his own soul for the sake of casting his last Wild Magic spell, he will revert back to being an unintelligent Zombie Dragon alongside his undead minions. In other words, if an undead creature was roaming around without a soul attached to its body, then it is classified as a mindless husk walking about randomly until it is killed in combat. This would explained the disastrous event which occurred in the fallen country, Inveria where all its citizens ended up becoming low-tier zombies. Their sudden deaths were due to the work of Cure Elim's Wild Magic spell gone wrong when copied and used by Keno Fasris Invern through her Talent. Without a soul, Inveria's dead countrymen are immediately reanimated as low-tier zombies. Based on that catastrophe which took place, the nation is now shrouded in vast negative energies where soulless beings are walking corpses. Moreover, it also became a spawning ground for stronger undead to appear among the low-ranking ones over the course of time. Sometimes, a naturally-spawned undead may turn out to appear intelligent like Davernoch, suggesting that this kind contains a soul for it not to seem mindless. In that regard, Elder Liches spawning from the corpses of evil magic casters.

Trivia

 * According to the author Maruyama, Ainz's Book of the Dead which was given to Fluder Paradyne is primarily about the departed souls.
 * After the player, Momonga arrives into the New World as an undead, it was said by Maruyama that his soul/mind became greatly warped as a result of it.
 * In the Web Novel, when Ainz used the World Item, Avarice and Generosity to store experience points from his slain enemies, eye-witnesses viewed it as though "souls" are being sucked up and taken away during the process.
 * The author Maruyama questions the outreach of Neia Baraja's power from her Evangelist class whether it can effectively influence the departed souls after Roble Holy Kingdom's war with the Demi-Human Alliance was over.