Generally considered by many to be a combination of sulfur and rotting fish. Theories abound as to whether this is just an inherent side-effect of evil (like an evil gene or pheromone), or a conscious choice of evil not to bathe very frequently.

On the whole, this works to the advantage of good; you can smell evil coming a mile away. While evil may take on pleasant form, even seductive form, it can never seem to shake off the smell.