@FrettzoOr you could make horrible soul-demon amalgams!
I've got an idea. Not only does a half-returned soul not contain the aspect of the element its missing, but it also feels a constant yearning to make up for this. To a simple necromantic soul, this might express itself through hopeless longing; a soul without Shadow might surround itself in knick-knacks and souvenirs from everyone and everywhere it visits, while a Soul without heart might surround itself with life.
But when you mix in a demon, the undead creature becomes much stronger, the same way concrete does with rebar. Its hunger also becomes insatiable. They feel a constant urge to fill themselves with something, anything, so they kill mortals and consume what aspect of their souls they're missing. But this isn't enough to keep them whole for long; perhaps the soul cannot ever truly be taken, and eventually Reathos takes it back from the amalgam's stomach, or it simply floats away.
Sound like something good?