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 different fuels for the successful casting of both Resurrection Magic and Wild Magic respectively.

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 the living 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. They could even consume the souls of the dead for sustenance, growing stronger the more souls they ate each and every passing time. One of YGGDRASIL books, Book of the Dead was about transforming the dead into souls. With YGGDRASIL origin, it details how souls are entities like the foam left by the waves of the New World.

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.

From Neia Baraja's knowledge as educated in the Roble Holy Kingdom, once a person dies, their soul would return to the great flow, where the gods would judge her. Those who accumulated good deeds as written in the scriptures would go to a land of eternal rest, while the wicked would be delivered into a land of torment.

According to the priests' religious viewpoint in the Baharuth Empire, souls resided within the body of every living being in the New World. A fleshy human body for instance, served as nothing more than a mere vessel to keep the soul contained in it. When any one of these bodies begins to rot, that soul would be summoned away by the will of the Four Great Gods. That was why the dead had to be buried immediately, with underground burial as the norm, but it was slightly different for those with special status such as the nobles. Hence, the place where the corpses were laid to rest was the space of the communal crypt or the noble's family crypt. After placing the bodies there for a period of time, the priests would judge if the body was rotting and the soul had been summoned by god before burying it.

Other than this religious principle shared among priests, they theorized that the undead entities are responsible for the corruption done to the souls of the dead. Temple of the Four Great Gods are looked upon by its religious followers as the saviors of peoples' souls.

While so, native denizens of the New World would solemnly clasped their palms together to respect their fallen loved ones, praying for the souls of the dead during ceremony-type memorials or burials.

In accordance to the Re-Estize Kingdom's education teachings of divine magic, the human soul spiritually required a physical vessel to inhabit and take control of. Destroying a skeletal corpse that lack its original flesh and blood, every nearby human country except the Slane Theocracy had the same consensus similarly that such an act is most likely to also get rid of the former vessel of the soul. In others words, they needed to dig up old bodies or bodies that were turned entirely into bones and proceed to break them into pieces.

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. Cure Elim's most potent spell was the type that destroys the soul of its enemy, preventing normal resurrection magic from being able to revive them without it intact.

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 woman as part of that individual's deal before laying claim to their soul. A Succubus is a prime example of consuming the souls of men through sexual acts with their soulmate. Devils can even steal souls from the individuals they made contracts with accordingly as promised. In addition to that, Evileye noted how Demiurge's [Hellfire Wall] 7th tier spell focus on burning the foe's soul rather than their flesh and bones. Similar to Necromancers or undead entities, this suggests that other demons apart from Demiurge who have the power to negatively affect the souls of their targets. Particularly, the demon Overeating could consume the souls of foes to reproduce their victims' screams that would terrify and kill any other living creature out there. This suggests that the demon's abilities can go as far as to induce fear to a person's soul if not protected with items or counter spells. One of such demonic abilities is [Soul-Bought Miracle], has special origin hailing from YGGDRASIL. It was the kind that happens to be inspired by stories of selling off one’s soul to the devil for the sake of fulfilling one's wishes. It was believed that conjuring forth demons from the Demon World to the New World will stain the summoner's soul. Likewise, the same could be said with a person creating undead creatures of death which hated life, was an act which defiled the soul as well.

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.