Force-of-Nature villains embody pure evil, or chaos, or other forces that move the world beyond the hero's control. They're not traditional characters with motives, desires, and a backstory that makes it easy to understand them. They exist purely to drive an agenda. The force they represent (and push relentlessly onto the world) defines the hero's values by interaction, giving the hero's human flaws and virtues greater value when contrasted with the villain's single-minded purpose. These villains are frequently portrayed as gods or monsters, like Sauron, but when written as humans, like the Joker or the Illusive Man, they are usually written as insane or obsessed.
