Souls

In the old world, before the end, Souls were a metaphysical manifestation of a being's essence. Now, with every heaven shattered and every hell overturned, that's only true for a time. Souls refer to that immortal essence within every living (and some dead) creature in Ithekshem, but also the coagulated sludge of many dead souls mixed together after they've been dissolved and any self reduced to screaming fragments.

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Sometimes, the divide is called Souls and Soulstuff. Either are potent forces of magic for those willing to manipulate and potentially destroy the last echoes of a person, with souls being more difficult to control and diffuse, but a more coherent source of energy.
In addition, Souls offer more than just raw energy - many of the dead have knowledge or information to share, charming personalities to engage with, or perhaps an enemy whose crime are severe enough that mere death is not enough. Undead raised with a whole soul are generally the same as they were in life, being less so the longer they've been dead and the more of their soul that have had time to dissolve.

Soulstuff, on the other hand, is a amalgam of many souls that have all but dissolved. If every soul is a drop of water, Soulstuff is a lake or river, every person dissolved into a greater whole, a uneven mass of broken used-to-bes. Though invisible to the naked eye, such gatherings of power are obvious to even the most magically deaf - hairs stand on end, voices whisper, an overwhelming sense of mortality grows in such places and etches itself into the very landscape. The more Soulstuff gathered in a place, the stronger such effects become.

Corpses in the vicinity of gatherings of Soulstuff tend to spontanously reanimate, and such wild undead cause no end of havoc.

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Although Death Magic is most commonly associated with magic that manipulates or consumes souls or soulstuff, other schools find equal use to them. Astral affect the soul of the still living, and can perform both surgery or mutiliation on the souls of the dead, too. Blood Magic sometimes claim more than just the life of their victims, and combination of the three schools with any other can cause a number of unpleasant or useful effects. Souls, as a convenient if gruesome source of raw power, is often used by the ruthless to create relics and artifacts - but not without side effects. The stronger the soul burned as fuel, the more likely its personality is to imprint on whatever is made.