Because the Modder can't be bothered to explain it:

The zombies are known as "The Omega", you need to setup them up via advanced setup (or set yourself as this to play them)
You need to turn on complete kills

And this is what happens:
The zombies will spawn around 1800-2000 (year), their arrival is not annouced normally (if they spawn from a capital it kinda is but not intent)
The zombies will instantly kill a target if you fail to kill the zombie while defending and multiply
Use of Forts and Citadels prevents this^
The zombies 1-shot Cities (destroyed in one hit, not captured)
Saving and Loading will break the mod

For those who want to fast-run this mod, start up a game in Modern Age and it wait about 10-30 turns for sure-fire zombies