Villains

As both a purveyor of sci-fi/fantasy and a creator of D&D campaigns, the concept of the villain is one I'd had little choice but to think about. What makes a good villain? What makes one frightening? How does one create a character who is believably evil or, at the very least, someone worth stopping?

Mostly, though, I wanted an opportunity to talk about The Master and Moriarty, and why I love them so much.