Demons are beings of pure evil and chaos. They are thought to feed on suffering, emotions, and collapse. Just like Gods, they cannot cross the Veil of Concordance, but their influence leaks through cracks: dreams, whispers, cursed rituals, and perhaps even the Crimson Mists.
Most demons are mindless creatures, bent only on destruction, but the most powerful of them - often called arch-demons - embody emotions, similarly to the Gods, and have some semblance of control over that domain. Arch-demons seem to act with purpose, cruelty, and cause.