The Empire racked with the Rat Plague forcing the Empress to send Corvo the Lord Protector, her own very bodyguard to neighbouring city-states in hopes of a remedy to this situation. After many months, Corvo returns home earlier than expected with nothing positive to share with the Empress and find himself at the wrong place at the wrong time.
Have you ever thought that the world has in fact too much goodness and order within it, and simply doesn't bare enough chaos, death or destruction in your day-to-day life? Don't worry, you're not alone in this strange world as a group of demented gremlins have the exact same thoughts so they revived a malign figure who was taking a nap. All malign figures need a nap every now and then.