This is based on the Rome total war 2 mod Constantine:Rise Of Christianity:https://steamcommunity.com/workshop/filedetails/?id=299700707
To form Rome you need to conquer all the other emperors and have Rome as your capital.
Each Major Civ needs to win by capturing a bunch of provinces there they can form there special nation.This applies to
Download:Constantine Civil War-20190101T204723Z-001.zip
(Installation put all the civs into the civilization editor and put the scenario into the scenario folder