Maxwell's demon, created by James Clerk Maxwell to demonstrate how the second law of thermodynamics might hypothetically be violated, has sharpened our understanding of the asymmetry of time (in thermodynamics). There also exist demons revealing the breakdown of other symmetries in our nature. In this seminar, I will talk about the demon in Maxwell's own theory of electromagnetism, by pointing out the asymmetry of electricity and magnetism, i.e., the breakdown of electric-magnetic duality, in the full quantum theory of electromagnetism.