Soul

The soul was the lifeforce of a mortal, the part that gave essence and a separate existence to a creature. In some religions, this lifeforce was variably termed a "spirit", and the two terms were synonymous. In the Mulhorandi faith, this lifeforce was termed two separate spirits, ba and ka. The people of Sokkar referred to this lifeforce as ka. And dragons called their lifeforce an anima.
  When a mortal died, its soul and body were separated, and the soul automatically began a journey to the afterlife. In the Mulhorandi faith, the ba spirit was said to remain within a mortal's body when they died, while the ka moved on to be judged by Osiris.
  In contrast, the case was different for some other kinds of creatures that did not have such a dual nature. For example, some creatures, usually from other planes of existence and sometimes referred to as outsiders, did not have a dual nature. An outsider's soul and body formed a single spiritual existence, tied fundamentally to the essence of its plane. If an outsider died, no soul left the body. Instead, its essence merged with the plane.[8] Moreover, when such an entity traveled to another plane, it remained anchored to its original plane with a mystical silver cord. If the visitor to another plane was killed on that other plane, the silver cord would pull the essence of the creature back, where it would reform into the creature it once was given enough time.
  Most plants, oozes, and vermin were not sufficiently sentient and thus did not have souls. Undead and constructs did not usually have souls either, though the creatures from which they were constructed might have.
  Primarily, any creature that would go to an afterlife has a soul. Most sentient creatures have a soul, but sentience does not guarantee a soul, such as elementals who are the spirits and embodiments of their respective elemental plane, or sphinxes. Some descriptions in various places imply that all mortal creatures have a soul, but there is no clear answer. It may be possible for adventurers to seek out and talk to the souls of friends by travelling to the afterlife. It may even be possible for the soul to travel with adventures for a time, though whether they’d want to leave or even be able to leave the Outer Planes is something to consider. @dream
  Gods and greater powers want to collect souls for various purposes: hags collect souls to gain absolute control over that creature, devils use corrupted souls in their hellish armies, some good gods want to grant their followers eternal life in paradise, and some evil beings consume souls for power.
 

You’ve Met With a Terrible Fate

The Outer Planes are not the only fate for souls. Sometimes, misfortune can befall someone and prevent their soul from reaching their desired afterlife. Here’s a list of some of those.
 
Fiendish Fates

 
  Devils barter in souls, and when they claim a soul, they drag it to the Nine Hells to become a lemure, twisted and tormented. However, lemures have the ability to be promoted to higher forms of devils with more power and freedom, and even have the potential to overthrow Asmodeus and become the ruler of The Nine Hells. Tiamat can turn her followers’ souls into a devil called an abishai. Certain souls can skip the lemure stage and be transformed into a different devil, such as fallen soldiers becoming merregons, fallen paladins becoming narzugons, and worthless, wasteful souls becoming nupperibos. A body possessed by a demon is at risk of having its soul dragged down into the Abyss if the host dies. Souls that are shunned or cursed by the gods are sent to the Abyss where, if they can survive, they are turned into manes. Manes can progress in power to become a stronger demon, even a demon lord, if they are able to survive being destroyed by other demons. Trapped or Traded Night Hags corrupt souls and then harvest them to bring them to Hades for their own purposes. Perhaps they use them to barter with devils, or perhaps they use them in foul magics. Undersea powers may bargain with sailor’s souls to avoid death, either caused by accident or by the undersea power itself. These souls are transformed into the horrific creatures called Deep Scions. The fate of these souls after these transformations is unknown. Undeath Souls can be prevented from passing, causing a wide variety of undead, such as:
  Specters: angry, tormented souls doomed to never pass from the Material Plane. Their only end is the oblivion that comes with the destruction of their soul. Can be formed when a wraith rips a living soul from a body. Revenants: the soul of a mortal creature who met an undeserving end and clawed its way back into the world to seek revenge. It has one year before it must pass on to the afterlife. Ghosts: a soul bound to haunt a location, creature, or object due to unfinished business it had in its life. Such business could be avenging its death, fulfilling an oath, relaying a message, or destroying an entire family or organization. Mummies: a soul bound to its body through a necromantic ritual after its death. Some mummies are created as punishment for their misdeeds in life, and some are created as guardians to tombs of power rulers. Mummy Lords: similar to a lich, but instead of binding its soul to the mortal plane, its soul is prevented from passing with dark rituals. Vampires: when transformed by a vampire, the creature’s soul is trapped in its undead body until the body is destroyed, upon which its soul is released. Most vampires create vampire spawn, which are under the total control of the vampire who created them. Will-o’–wisps: created by the death of evil beings that perished in misery or anguish in forsaken realms steeped with powerful magic. Boneclaws: A failed lich transformation. When the aspiring spellcaster is too weak to compel their spirit into the phylactery, it insteads finds the nearest evil soul and binds to that instead. It then manifests as a boneclaw, bound to the evil creature until either the creature dies or turns away from the path of evil, upon which the boneclaw soul is destroyed. Deathlocks: created out of the soul of a warlock who failed to live up to their end of the bargain. Eidolons: forged from a fanatical soul to protect the secrets of gods. Sometimes granted as a special reward to followers. Sword wraiths: the soul of a glory-obsessed warrior who died in battle without earning the honor it sought. Lich: a powerful spellcaster who escaped death by binding their soul to the Material Plane. The lich must consume souls to feed their else they will wither and perish. Banshees: female elves can have their souls trapped through the banshee’s curse if they use their grace and charm to corrupt and control others. Death Knights: fallen paladins who, after death, are transformed by dark and malevolent powers. They can only escape undeath by atoning for their misdeed and seeking redemption. Wights: people driven by dark desire can plead to a dark power, such as the demon lord Orcus, to escape the afterlife in return for waging eternal war on the living. Destruction Liches consume souls to feed their phylacteries. Wraiths are formed when an evil soul is obliterated. When an evil soul has been suffused with enough negative energy, instead of their soul being shuttled off to a terrible afterlife upon death, they implode and cease to exist, leaving behind a wraith. The Negative Plane will often obliterate the souls of those who visit it, leaving a horrific monster known as a Nightwalker in its place. A soul can meld with primordial matter, destroying the soul but creating a genie in the process. The genie may have some memory of the soul that created it, or it may have no connection to it.