Crystals

It's common knowledge around artificers, wizards, and the like, that every living thing has a small amount of pure magic in them, no matter how small an amount. And further few, yet still many, have with them the knowledge that after the death of said living thing, that same magic seeps into the ground, coming together to form crystals of energy; while the soul of the thing meanders on up to whatever afterlife exists for it. (it is a large debate among magic folk whether trees and plants have a form of soul, and where it goes if they do.)

However, the case with people is different.

if someone were to desire this magic, they could do so by killing another living thing- be it creature or plant, the magic would be passed on to the man who killed it.

But when that person dies- and their wrong outweighs their right- then their collected magic, when returned to the ground, will take their soul with it. If the resulting crystal has enough energy for it- which most of these soul crystals do- the soul trapped inside can and most certainly will feel the anguish and pain that you would expect to come with being trapped inside a crystal for eternity. The more magic energy is in the crystal, corresponding largely to the lives taken by the soul, the higher the intellect of the crystal (which proves useful to artificers), and the more anguish felt by said soul.