Soul

The seat of the body is the soul.

— Ardaenian proverb

Every sophont in the Multiverse has a soul. The various religions and philosophies of the worlds might question its role, how it got to be there, whether they are immutable or not, etc. But none disagree about the fact of their existence. The reason? A great many demons or necromancers would love to buy or steal them.

Indivisible Unit of Energy

A soul can only be harvested from the recently-deceased corpse of a sophont. If the body has been dead too long, the soul has dissipated into the ether or - much more likely - has already been taken away by something else.
The soul isn't exactly physical. It would be better described as a form of energy that can be captured, like the energy of a wave. Various sophonts and demons have created technological or magical devices capable of holding onto soul energy or converting soul energy into other forms.

Why do they have to be dead?

There are three major reasons why sophonts need to be dead in order to harvest the soul. First, a living sophont tends to walking, talking and generally using the soul still, so removing it would be rather jarring. Second, the reason soul energy is easy to harvest after death is because it is readily available; trying to collect before death would involve a great deal of energy and effort, possibly more than the soul itself could provide. The third reason is a little more terrifying, as this anonymous scrap in the Endless Sophos can attest:

These Soulless Ones are a walking nightmare. Anything civilized about them, any sense of language or culture or consciousness, has vanished. We had to put them down because otherwise they would kill and destroy anything in their path.

CW: existential horror

Type

Metaphysical

Related Sophonts

All sophonts have a soul by definition.

Appearance

Incredibly variable

Who Wants Souls, Anyway?

Demons from the Tortuous Fire and Fay tend to be the main demonic purveyors of soul energy. Fire Demons tend to collect souls by making bargains or transactions with sophonts, gaining their souls in exchange for giving the sophont some kind of power or influence. The Fay tend to steal them, claiming the soul as property when the sophont has "mistreated" them by not following their rules.
Necromancers - mostly famously the Vitlesh of Dreandril - also deal with souls. Some of them are interested in preserving the soul, either to return into the dead sophont after they are regenerated back to life or to transfer into a new sophont body. Other necromancers use the energy of souls to preserve their own life force, lengthening their lifespans or reversing the effects of disease.