When someone on Morus dies, they become a petitioner. What comes next varies. Shortly after their deaths, they usually hang around in the ethereal for a while before they are collected by one of the agents of death and ferried off to their destination. During this time, if they were followers of druidism who managed to achieve at least some union with Nature, they will disappear into the Animus, becoming part of the great lifeforce that drives the multiverse. Otherwise, they will be drawn from the mortal coil and to the outer planes as a petitioner.
There is a difference between a petitioner and a planar. The former is a disembodied mortal soul, while the latter is a spirit. Petitioners are immortal while on their home plane; if slain, they will simply reform a short time later. Nothing is lost, although their experience points reset to the beginning of their current level. However, if they leave their home plane, they are at all times in mortal peril; for outside of the realm of their alignment, being slain will result in their complete and utter destruction.
Planars are the opposite; creatures like angels, demons, and so on. A planar who leaves his home plane does not need to fear a permanent death. They are a part of their home plane - in a way, they are almost like an extension of it. Only by being confronted and slain on their home plane can their will and personality be permanently destroyed. Otherwise they will reform, although this may take considerable time.
It should be noted that there is a third type of being on the planes; the non-planar. These are neither petitioners nor planars, but are mortal beings that simply dwell in the Outer Planes. A planehopping elf from Morus, for example, does not suddenly become planar; if they die, they die. If a human dies in the Outer Planes, their soul still goes to their designated afterlife - and so on.
Assuming that they do not discorporate into the energies of the multiverse, as Druids do, a mortal soul tends to hang around the Ethereal Plane for a short while after death. The period in which they remain in this fashion is indefinite; some strong-willed souls will not depart at all, choosing to remain in the Ethereal as incorporeal undead. Others, sadly, will not survive to their afterlife - a defenseless soul that wanders from the Border Ethereal and becomes lost is likely to be devoured by one of the many incorporeal predators of the spirit world.
When a discorporate soul is ready to enter the next stage of their existence, they will be approached by a psychopomp - the physical manifestations of Death. Then, they will be conveyed to their respective afterlife via the secret paths that only psychopomps can take into the Astral Plane.
Life as a Petitioner
What happens to a petitioner after they are conveyed to the Outer Planes varies. In all cases, they slowly lose the memory of their past life - after a few months, only their core personality and mannerisms will remain. Most petitioners retain the appearance that they had in life, but not all - some are drastically transformed. If they are not transformed into a "special" kind of petitioner such as an einherjar or an archon, they lose all of the power and skill they possessed in life - effectively returning to level 0.
It is, of course, possible for a petitioner to "relearn" their skills, advancing in level and growing stronger. This is relatively unlikely, however, as most petitioners find spiritual development is far more attractive than the aquisition of raw power in the afterlife. They are, after all, in paradise - what is there to strive for but the ultimate refinement of the soul?
If a petitioner worshiped a particular god who accepts followers, and was true to their deity, they will usually be brought into their deity's realm in the afterlife. Otherwise, they will go to the Outer Plane that most closely matches their alignment. Certain places - such as Hades, Carceri, and the domains of certain gods - will trap the souls of those who die there, preventing them from going to their proper plane. Mortals who die in these places will instead become petitioners there.
As noted above, not all petitioners simply become 0-level versions of themselves in the afterlife. In some special cases, they might be transformed drastically. Some of the better-known variants of petitioners are given below, but it should be kept in mind that many other variants may exist. For example, the petitioners of a fire god may become lizard-like creatures who are immune to heat.
Archons are Lawful Good petitioners who have gone to the Seven Mounting Heavens of Celestia. Not every petitioner who goes to the Seven Heavens become an Archon; it is usually only clerics and the very devout. In short, those who do not wish to simply rest and enjoy their afterlife, but desire continued service in the name of Law and Good are elevated. They begin as Lantern Archons, the lowest form, and work their way up the celestial hierarchy through service and enlightenment.
Bacchae are Chaotic Good petitioners who dwell in the first layer of Arborea. They are more Chaotic than Good; they are made from the souls of those who, in life, perfected revelry and the enjoyment of life above all else. Often a dangerous and unpredictable force, they are a whirling frenzy of drunken merrymaking that sweeps away anything in its path.
Einherjar are usually Chaotic Good or Chaotic Neutral petitioners in Asgard; although there are some other gods that create einherjar besides the Norse Pantheon - particularly gods of war. The einherjar are in most respects identical to ordinary petitioners; however, they retain all of their strength and skill from life. Levels, hit points, THAC0, spellcasting ability and so on are retained by the einherjar - although they have have a ghostly form outside of their home plane that marks them for what they are. Obviously, empowering and using petitioners as soldiers and warriors is somewhat risky for a deity - since they will be permanently destroyed if slain beyond the borders of their home plane - but many of the more warlike deities choose to do so anyway.
Chaos Beasts are those who have met with a terrible fate. They dwell in Limbo, the plane of absolute Chaos, where the Lords of Entropy rule and the Slaadi roam. Few become Chaos Beasts "naturally"; even those who are completely mad will usually be cured of their affliction in death. Only those who are truly and utterly insane to their very core would be consigned to this fate. Most Chaos Beasts, however, are the unlucky souls who were slain by a Chaos Beast in Limbo and became doomed to a life of formless, shapeless madness.
Larvae are Neutral Evil souls that were especially selfish. Not all Neutral Evil figures become Larvae, even if they have no god to protect them. Those who do, however, are doomed to a pitiful life. Larvae are the bargaining chips of the Lower Planes. They are traded and captured by Night Hags; they are used by liches to sustain their immortality or by evil wizards to fuel dark rituals; and demons and devils use them to create quasits and imps, respectively. Because imps and quasits later advance to become greater fiends, Larvae are one of the primary foundations of the Lower Planes.
Manes are the lowest of demonkind, and the only rank amongst the Tanar'ri that are petitioners rather than planars. Those who die and are Chaotic Evil are doomed for this fate - a mindless, ravening demon of the least order. Thereafter, their most likely fate is to have their life force consumed by a more powerful demon. If they are lucky, however, they may distinguish themselves by consuming other manes, and eventually become a true demon. There are only two other ways to avoid this fate: one is to seek the patronage of a powerful god of evil, who will claim your soul after your death. The other is to commit acts of such frightening malice that you are judged accordingly by a Nalfeshnee and transformed into a more powerful demon after your death.
Lemures are the devil equivalent of manes, but are possibly even more pathetic. They are amorphous, tortured blobs that are the eventual fate of those Lawful Evil souls who die without an adequate safeguard. They do not even have the hope of a manes, of promotion by cunning and ferocity: a Lemure is only ever promoted by random chance. The only way for a Lawful Evil mortal to avoid this fate is to serve some dark god who will take your soul under their own protection in the afterlife.
Nupperibos are another Lawful Evil petitioner, also found in the Nine Hells. Retaining more of their human shape, Nupperibos are made from those Lawful Evil individuals whose souls were not even dark and malign enough to become Lemures. Although they have a higher station in the Nine Hells due to their willingness to follow orders, they are blind, deaf and mute - and they cannot ever advance without first being "broken down" into a Lemure. Each time a Nupperibo is slain on the Lower Planes, there is a small chance that this happens.