Golden Horde Arrival

I am sure this has been asked before but i cannot seem to find it.Is the arrival of the Horde hard-coded or can we edit it.The reason i ask is that i would like to see if it is possible to make a mod based upon Raymond E Feists SerpentWar Saga with the invasion of the western kingdom happening after a set period of time.

The date is hardcoded and you can't make "new" faction appearances, but you could use the Golden Horde and alter it into the faction you want it to be. The only problem will be if you had more then one faction in mind.