Imperium is an infernal energy produced by the orcuses in various parts of the Hells, but especially the Blackiron Ranges in the depths of Inferno. Users of imperium are called Refreno.

Blackiron, imperium in the form of a metal, when forged it creates an aura of authority or enslaves those caught in it
Ritual Castigation, using rituals of torment on others to make them soul-controlled slaves
Ars Imperium, alchemically wielding imperium, usually to create potions or philters that will make others your slave