Magical energy exists in three forms called divne, neutral and nether energy. Energy, in one form or another, is present in literally everything in the universe and can be utilized to affect and manipulate other energy, and matter, by casting magic.
Some magical activity happens naturally and automatically - For example this kind of process is what powers Heaven and Hell. It's possible for living creatures to learn to manually cast magic in various ways. However, many of these uses can upset the balance of things and have negative consequences.
Types of energy
Neutral energy is the energy of the mortal worlds that is all around us and within everything. Everything in existence contains this energy: Living creatures, plants, metal, water and rocks alike. Yes, even the air or the empty universe. Literally everything.
If a creature, or enchanted item, for one reason or another should let out some of this energy, lets say by using it to make a ball of fire, it will slowly regain the lost energy from its surroundings.
A creature can regain neutral magic faster by absorbing the energy of other things. If you are low on neutral magic, eating and drinking will help you recover faster.
Nether energy is what sins and Hell is made of and runs on. While divine energy is only accessible if you are an angel, and neutral magic can only be recovered slowly, nether magic comes in endless supplies. Sadly it also comes with side effects like mutation and loss of sanity, and that’s just the effects you would feel personally.
Unlike Heaven, Hell is more than willing to share its energy with everyone. It will often lend nether energy to you even if you didn’t ask for it. Such instances could be if you ran out of neutral energy, yet continued to fire off your fireballs. If that was the case, you would switch to use nether energy, most likely without knowing so.