We need to figure out the best way of going about the challenge factions scripts. Obviously some of them will happen later in the game and some earlier, and they will take different forms. The main dilemma will be changing settlement ownership since there is no direct way to do this and it will therefore require a roundabout method of it with forced sieges.

I think the best place to start is with the Mongols, since their challenge faction is readily apparent and will be from the beginning. For the AI, we need the mongols to retain all of their 'initial' settlements so they aren't impeded by only owning one province. For the player, we need the challenge faction to own all but the capital province.

The regions the mongols own(city is the same minus _Province):

Mongol-Omuk_Province (Capital)
Taichud-Omuk_Province
Jalayr-Omuk_Province
Tatar-Omuk_Province
Kerait-Omuk_Province
Naiman-Omuk_Province
Merkit-Omuk_Province
Kyrgyz-Omuk_Province
Tuvan-Omuk_Province
Qara-Qorum_Province

As a player what we want is for all but Mongol_Omuk to invariably enter the hands of the challenge faction which will represent the other tribes Chinggis needs to conquer, in the first couple turns. We can't use give_everything_to_faction and start it as challenge because that is a one-time solution and it would mean no other challenge factions in the game can start with provinces or characters.

The one good thing we have is that we can work fluently with characters since we will know all of their names, and can thus kill/spawn them where we want for who we want initially.