Zombies!

 

No good classic feeling RPG would be complete without at least a few zombies. Often over-looked as are kobolds and goblins, zombies may not be getting a fair shake in your campaign.

Take a look at all the zombies on TV and in the movies. They are an infection, an inescapable plague that can ruin entire civilizations, but in RPG's they are often a simple level one or two monsterthat gets hacked and everyone moves on, unphased.
Re-think the next zombie a little more, give them a bite that makes more zombies.... seen here, what if the water gets infected?
Keep players on their toes by using the basic creatures to their full potential.