Soul Magic and the Soul Cycle

All magic comes from the soul. From burning it, specifically. In severing its ties to material reality and releasing it back to the Sea of Souls from whence it came. This makes it somewhat difficult to use magic with your own soul without great personal danger, so the Cunning Arts have been built upon using the souls of other being.

Universal: The strength of a soul is equivalent to the number of HD the creature has. Spells are a limited resource with usage die.

Method 1: Knave
Each HD of the soul increases the usage die of the spell by 1 step (ex. 1 HD = d4, 2 HD = d6, etc). Use Maze Rats spell generation of Beloch's magic words for more fun.

Method 2: Typical OSR
As above, but in reverse: the die size decreases with the level of the spell. Spell level is determined by target HD (Level 1 = d20, Level 2 = d12, etc)

Method 3: GLOG
Each soul has spell dice equal to its HD. These dice burn out on a 6 as normal, and are not replenished.

A soultrap is a sort of lantern built of celestial bronze and finely-shaped lenses. It costs 1000 sp to purchase and is only available in urban areas.To capture a spell, simply open up the trap as the target dies. When a soul is fully burned, a new one may be trapped in its place.

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Stages of the Soul Cycle

The Sea of Souls

The source of all souls, and thus of all magic. Numinous and unimpeachable in its existence without form or place. Saints and scholars have yet to come up with anything but illustrative metaphors to define its relationship with the material world.

Embodiment

A soul that has been integrated into a material form. All living organisms have some sort of embodiment, though not all are suitable for usage as a spell (it would be difficult to do anything with the soul of a bacteria, after all). The soul-bearing properties of non-living matter are a still a matter of debate and study - do all stones have a soul, or only those worked by human hands? Do cities have souls? Do rivers? What if you move water from one river to another? So on and so forth.

Working magic from non-living souls is difficult, so most thaumatologists will focus on the living.

Undead

The end result when a soul is removed from an organism before its death. Biological functions continue apace, but without the soul's guidance its behavior may become errant, spiraling out into bizarre and inexplicable repetitive actions.

Ensouled

An experimental form of undeath where a soul is removed from an organism and then returned to it. Most are returned to their original bodies, but transference of a soul to a different body. The souls used in liches do not integrate as in normal embodiment, but are more akin to inhabiting a shell.

Weak Embodiment

An embodied soul that has been tapped for power without death. The targets of tapping are made pale and listless by the process, and recovery may take months or years if it occurs at all.

Strong Embodiment

A soul that has absorbed other souls, generally through consumption of a freshly-removed heart. There is something innately corruptive about this process, and those who practice it are monstrous indeed. No material form, it seems, is meant to hold more than one soul.

Ghost

Most ghosts contain nothing of the original being: the ba often evaporates instantaneously upon death. Those of potent charisma in life are able to maintain continuity into death, however, and even act independently of the typical repetitions of their actions in life.

Weak Ghost

The ghost of a weak embodiment. At this stage ghosts cease to have any individual traits at all, appearing only as misty, vague forms incapable of all but the most basic manifestations. 

Demon

The ghost of a strong embodiment, a roiling gestalt of misshapen and corrupt souls. The means of its creation has made it monstrous, and it will not submit to a soultrap without a fight.

Shade

An ashy smear of a soul - a ghost that has been burnt away to near-nothing, the last vestiges of a spell. The natural decay of a shade leaves it unstable and dangerous - the nuclear waste of the soul.