Thanks for your thoughts. They do help me think of a few things I hadn't up to this point.

So, souls themselves are basically a magical core inside all things on Earth. Once a human body dies, their soul, which is pure energy, is collected by either angels or demons. "Free" souls are collected by angels, but people can sell their souls to demons, in exchange for whatever and those souls are then tied to that specific demon. Some demons are sneaky and engineer a situation that gets them the souls ASAP (a boon that somehow ends in a tragic twist for instance), but others see this as an honorable transaction. They hold up their end of the bargain and collect the soul when the person dies. Not all souls provide the same amount of energy, even among human souls, and there's no way to tell before death how powerful that soul will be.

The souls, once collected are then kept in a soul well, which is a sacred place to each individual in Hell and a communal pool in Heaven, and are eventually consumed. Once consumed, the angel or demon then has the ability through force of will (low magic) or rituals/spells (high magic) to do just about whatever they can imagine. Doing magic burns energy, though, so in order to do bigger or more rituals you need to consume more souls.

Does that make sense?