ok to put it simple there is this game called myth it is probally one of the founding games that made this game possible.
If you like the total war series i highly recomend you search for this game, get myth II because you can download myth I and play with the 2nd.

http://www.youtube.com/watch?v=wjN6k...eature=related

its about undead and living units fighting. the undead have more numbers and are wiping humanity to extinction... ok here is a youtube link to the intro to this game.

I should also add that this game was made by bungie the same guys as halo and that the game still has a strong multiplayer community online going 10 years strong including myself and about 250 other dedicated players.

also here is a wikipedia article on the units that are in it. Some serious stuff to do here but would probally be the best mod for ages.
http://en.wikipedia.org/wiki/Myth_(computer_game_series)

this also has a seriously epic story, and has quite a history, the movies were actually made by disney belive it or not!