Soul

The soul is the fundamental building block of life itself.  Sure, carbon is important, but all of the physical trappings of existence pale in comparison to the power and value of the soul itself.  It is that which infuses sentient creatures with the capacity to make decisions both large and small, and is the component of live that passes on to other planes or new bodies at the time of a creature's Death. 
In some ways, the soul is a very simple thing.  The above paragraph describes everything the average citizen needs to know on a day-to-day basis.  The details, however, can be complex and intricate.  Most notably, the actual origin of souls is still completely unknown.  No amount of  science, magic, nor divination has been able to determine where precisely the soul comes from; it simply appears at the time of a creature's birth, as if from nothing.  Some scholars have recently theorized that they may be coming from The World of Forms in some manner or other, but critics say there is no evidence to actually support this claim.  A wide variety of other theories have been proposed, each just as unfounded as the last; perhaps they come from other souls in a process similar to mitosis, perhaps they come from the moon, or the stars, or the future, or the past, or from another plane that is as-yet undiscovered.  Souls have sometimes been observed to reincarnate into new creatures, however even this phenomenon is not universal and does not explain the fact that the total number of souls in the multiverse is steadily increasing.
What is known about the soul is that it is incredibly hardy.  Even a god cannot destroy a soul, though some have found ways to diminish or transform them.  They can be driven to madness by the River Styx, forcibly reincarnated into new bodies that are lesser than their old ones, chained, trapped, or even tortured.  Actual damage, however, is exceedingly rare.  It requires extremely powerful magic to cause lasting damage to a soul, and even this will eventually repair itself in time. 
The soul is also a constant wellspring of magical energy.  Gods draw power from the souls of their followers.  Some spellcasters draw power from their own souls.  Most healing and necromantic arts draw from the souls of either the one casting the spell or the recipient of the healing; drawing power from the soul itself in order to restore the vessel which contains it.  Even Conduit powers are theorized to harness the soul in some fashion, though evidence on this subject is scant and experimentation is ongoing. 
In the relatively short timescale on which mortals typically operate, this magic and free will is what gives the soul so much significance.  On the much larger scales that gods and some other immortal beings work on, however, the primary value to a soul is as a source of labor.  A god that can sway a soul to their cause, through persuasion, intimidation, or even simple business agreements will secure a follower that can serve them for eternity in the afterlife.  The labor of a single individual is typically a minor boon, but when that boon is extended to the rest of eternity the true value of such an acquisition can become nearly infinite.

Item type

Magical