BI: Modding Emergence

[Epistolary Richard]

No, have a look in descr_sm_factions, it's defined in there. They're marked as spawns_on_event, spawns_on_revolt, shadowed_by etc. It seems that only the RB event (should you choose to use it) is relatively hardcoded.

But you do need to add a map and an image in order to make them playable. Have a look at this:
https://forums.totalwar.org/vb/showthread.php?t=55192