"Souls" & Magic

Within Sem, souls act as the peculiar middle ground between the material realm and the metaphysical realm, which both parallel one another. It may help to think of the soul of a person that physically occupies their body in the same way flesh does. As the material flesh rots, or as the metaphysical manifestation of flesh breaks down, the soul of the creature also does. The soul manifests as non-wave particles which travel very slowly unless excited by certain magics (read: necromancy), and as metaphysical energy within the metaphysical realm, which are both linked to one another; if this soul energy moves in the material realm, it will screw up the makeup of the metaphysical realm in such a way that it can cause metaphysical energy to travel through the soul and end up as a wave-based or matter-based element in the material realm.
While flesh itself acts as a physical anchor holding soul energy in place, it still exists within both realms as it does when the flesh has rotted and the soul is free to wander. When someone waves their arm, speaks command words, and channels with a foci, they are, essentially, screwing with the metaphysical realm in a way it was not supposed to be messed with, and the resultant energy is transferred into the material realm by the soul, where it is initially a wave-based particle, but it may afterwards be converted into matter through further manipulation.
As an example, when casting Create or Destroy water, and wanting to create 10 gallons of water in an open container, first the caster must hold their foci (A drop of water, or any casting foci such as a deep metal, or holknum) and begin channeling metaphysical energy from it to give the spell enough power to manifest. Second, the caster must perform the somatic and verbal components of the spell, which disturbs the metaphysical realm, and then this disturbance is amplified through the foci, which can then transmit through the soul and enter the material realm as a wave. The caster then finishes the cast by manipulating these waves which become water particles, and the casting is complete.
It is unknown how or why flesh is the only medium capable of holding in a soul, but it is possible to create false bodies of flesh for particularly powerful spells, however, it is generally less expensive to use corpses for magics, rather than to create entirely new and synthetic bodies which do not contain a soul.
Ghosts, spectres, banshees, and all other kinds of manner of undead, come about as a result of the particles of a soul bouncing into one another in such a way that different parts of the dead where the souls came from begin to form into a more cohesive shape. This occurs as bits of the soul "remember" where they are supposed to be in a body, and begin to attach themselves to other soul particles. Ultimately, as the whole body forms out of these particles, it takes on a grey metallic hue and gains consciousness. The whole process can frankenstein together parts of creatures that were not originally held together, causing the existence of the spectre to be completely original from another spectre. The abilities of a spectre heavily depend on the body's makeup, and the spectre will usually experience intense mish-mashing of memories, cognitive abilities, as well as numerous other aspects.
Necromancy as an art heavily deals in the manipulation of souls in order to create living corpses or spectres. Necromancy can also be used to re-construct lightly decayed bodies using other souls, or even particles from the body's own soul if they can be found. This is particularly useful if the person cannot be saved by resuscitation through medical or magical means.