Tampering with Creation itself to directly change the world was the reason God created Angels in the first place, to avoid Him harming creation with his unlimited power. When a demon performs such a miracle, it focuses its will directly upon the spiritual fabric of reality, just as it did when it still served God and used its divine powers to help create the universe. Almost any effect imaginable can be evoked through Arcana, from divining the future to calling down a rain of brimstone upon an entire city. Even the spells of a master mage pale beside the potential power of a demon’s black miracles. Limits exist on what Arcana can achieve, however, as well as to what miracles an individual demon can evoke.
Every demon has a specific area of expertise, a particular suite of Arcana that it can perform most skillfully. Almost any concept and feature of Creation can be the focus of a demon’s skills — from fire to deception, controlling ghosts to crafting weapons of war. In that one area, even a demon of moderate power can evoke exceptional miracles. Outside of that focus area, the skills of many demons fall off sharply. A demon might have two or three areas of secondary expertise in which it could evoke Arcana of significant but not extraordinary complexity. When using any other kind of Arcana, demons are limited to straightforward effects — and some minor demons may be completely unable to evoke some effects.
In essence, the Arcana are black miracles of demonic faith, which can imbue demon thralls. Through investments of Arcana, demons can restore beauty to the aged and disfigured, cure diseases and heal old injuries. They can grant wealth, prestige and supernatural power. The thrall really empowers himself through the proxy of the demon, trading raw devotion for the processed faith of Arcana.
- Devil's Due, p. 39