Every living thing is said to have energy. Non-sentients that breath their last, don't pass on to another system, instead they release a bit of life energy. Sentients on the other hand have their spirits depart for non-Mortal Systems. The destination is one of Alignment and patron deity. If they have no deity, they may spend a considerable more time wandering Limbo's dozens of worlds before they are picked up. If they don't fit any alignment, then they are waylaid in borderline systems like Acheron, Arcadia, Pandemonium, and the Twin Paradises until someone convinces or steers them to a particular Soul Gathering System. In these non-soul gathering systems the spirit is at risk of being captured by night hags and others that traffic in soul energy.
Soul energy is what was in the mortal body, with the spirit just a vessel and to those who can see into their spectrum, nothing more than a ghostly semblance of the creature when it was alive.
- Bruh Kreniik, from the book Wisdom of Bruh Kreniik - "Path of Souls"
In the earliest days of Creation, Bruh Kreniik searched for a way to make it so that when his creations died, they evaded Chaos and the process of being broken down and reabsorbed. Bruh Kreniik created the first Well of Afterlife, a device that served as a magnet for certain energy types, capturing the spirit before it was washed away in the Sea of Entropy.
In the God Era, the astral deva Polyxo experimented with soul energy at Quelmollendë. She intended to make a variant of the dwarves that was more accustomed to living in the deeper areas of the Underdark. Her experiments using soul energy to create new life forms were only a partial success. This was because the souls she used were destined for the Abyss. Normally, these souls would be the chattel of the system's demon lords, beginning anew as a pitiful mane. Polyxo re-purposed these chaotic evil souls, using them to create new life forms. For most of those created in this manner, the outcome was something degenerate, mutated, twisted, or prone to insanity. The Derro race were one of these.
Soul energy is also used for empowering magic items with sentience. This can be done either willingly by a good spirit for a good aligned item, or forcibly for neutral or evil ones. Some weapons steal soul energy to power themselves or to crush the soul into non-existence for truly evil ones. A soul destroyed in such a manner is very hard to remake for even the gods; those knowing this are thus aware that Higher Powers are not so omnipotent as the masses are led to believe.
When immortals die, their soul energy returns to their system of origin. A demon returns to the Abyss, a devil to the Hells, a modron to Mechanus, a lawful good celestial to Nawenglorûs, and so on according to their Soul Gathering System. Their life energy is left behind at their place of death. It is replaced as their body reforms on their home system. The time it takes for the body to reform varies based on the former strength of the individual with Higher Powers reforming in a matter of days but lesser types taking up to a century. Non-mortals can only be permanently killed on their home system, therefore it takes special measures to take out a formidable immortal in their home system.
The first and foremost thing you must do when seeking to retire an immortal is to lock down the area and restrict the foe's movement. This means using magic to stop teleportation and other inter-dimensional movement. The next steps vary on the foe, but keeping others from rendering help to their master is next on the list, followed by plentiful vials of Godhead and a few mass resurrection scrolls.
- Corin Reradrane, from the adventuring band Retirees - "Immortal Retirement"