Soul Energy

content with wandering Pandemonium

Every living thing is said to have energy. Non-sentients that breath their last, don't pass on to another plane, instead they release a bit of life energy. Sentients on the other hand, their spirits depart for the outer planes. The destination is one of alignment and patron deity. If they don't fit any alignment, then they are waylaid in borderline planes like Acheron, Arcadia, Pandemonium, and Twin Paradises until someone convinces or steers them to a particular Soul Gathering Plane. It is in these non-soul gathering planes that 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 is alive.
 
- Io, from the book Wisdom of Io - "Path of Souls"

In the earliest days of creation, Io found that the sentients he was making were returning to the Elemental Chaos with death. He had the Wells of Afterlife created to capture spirits and use their soul energy to create a new being. This new being matched the alignment of the Well of Afterlife.

In the God Era, an astral deva named 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 plane, and rise as lemures or some other petty fiend. 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 aware then that gods are not so omnipotent as the masses are led to believe.