Dominions 3: The Awakening/Nations/Ermor

Once upon a time, there was an ambitious, upstart human kingdom. This kingdom had a name strangely similar to that of a certain well-known human empire in a mostly harmless blue-green planet in a galaxy far, far away.

It had trained soldiers and steel weapons. It had sorcerers eager to test their powers. And it had priests who spoke of a new order, on in which their empire would rule.

New Faith (Early Age)
The history of the New Faith chronicles the doomed nation's embrace of a new god over the state cult of the Numinas, which is clearly a reference on the rise of Christianity in Rome replacing the ancestral Roman religion.

Broken Empire (Middle Age)
The "new order" has proven to be substantially more dangerous than believed, and not just for the enemies of the Empire. The dead walk the earth, while the living struggle to avoid joining them. The Thaumaturgs and priests of the empire have some ability to control the dead, but the situation has become even more unstable with the rebellion of peripheral elements of the empire, such as the new nation of Pythium.

The nation still has legions, but now they are composed of an unholy mix of the living and the dead, and the chaos is not helpful.

Ashen Empire (Late Age)
'''Braaaaaaains! Moooooore braaaaaaaaains!'''

The catastrophe has finally overcome the living. The land is blasted and dismal; the unholy energies over the affected lands now slay the living en masse. The dead rise in vast numbers to conquer, while all but the most foolish flee before them.

Inspiration
For the latter eras, try watching just about any walking-dead flick. Army of Darkness works especially well.