The demands associated with having a medium size zombie army helped define the antagonists in my current campaign. No single person could come up with that many monsters on his own. Therefore, the main villain in the story had to seek some support from the local populace.
This support did not come right away. It took years of work and the right circumstances to secure their loyalty. Once enough people were signed in, this new formed group moved forward with their plan.
Using the Mayan religion and customs, he preached a return to the old ways. His followers shared in his vision and started to take captives from the smaller tribes in the area.
Using an ancient artifact, they transformed the captives into zombies. This artifact is a tool of great power. Many warnings were known about it, but in his quest to banish the Spaniards, the evil shaman decided to forgo them.
Most of the captives possessed no innate magical abilities and were transformed into zombies. Those that did, however, were transformed into other undead types. These new types, while not as easily controlled as the zombies, offered the evil shaman with many options not available with the zombies.
The artifact also functioned as a gateway between our world and the hellish world were its power emanated from. This is the main reason most other shamans avoided using it.
Once the gateway is open, many powerful beings could crossover into our world. Fighting them is a matter of survival. One which is really hard to achieve with a being that is indomitable and evil.
This is what has transpired in my campaign so far. Using zombies was a good way to start things off and define the antagonists while also including several different types of monsters in the game.