From Diablo Wiki
Demonic magic is a magic source from Demons that's full of destructive power and danger. It has a strong corrupting influence on its users, and the forces of Darkness will in the end try to turn the caster into a servant of the Burning Hells. To fully grasp the concept of Arcane magics, it's good to have read about the general nature of magic first.
Diablo Game Demonic Magic
There are no classes in any Diablo games using Demonic magic, it is the force of Evil and everything any player character would want to fight. Throughout the history of Sanctuary, there has been a lot of cases of humans using demonic magic, usually ending in one catastrophe or another. There are "monsters" using these dark powers though:
- Cultist - Casting fire spells and summoning Dark Demons.
- Dark Vessel & Activated Vessel - Humans who have let themselves be completely overtaken by demonic spirits.
Nature of Demonic Magic
Demonic spells are simple in nature; they are either used to damage something, hurt/corrupt someone or to bring forth something that will in the end hurt someone/something else.
Other Schools of Magic
Arcane magic - Divine magic - Demonic magic - Godly magic - Holy magic - Nature magic - Spirit magic - Death magic - Life magic
Prime magic - Elemental magic - Fire magic - Lightning magic - Cold magic