The purest essence of the Hells. Demons are souls composed of infernum. There are many methods of wielding infernum:

Thelemic Ritual, using ritual to obtain demonic powers
Demonology, the study and practice of making deals with demons
Demon's Kiss, sexual demonic ceremonies
Hellfire, pure infernal flame from the Hells, used to burn souls

Using other energies to wield infernum or using infernum to wield other energies:

Demonomancy, using infernal energy to explore the paths of fate
Demonomagic, the intersection of mana and infernal energy
Demonokinesis, demonic psionics