Some things I concluded from looking at the code as well, hopefully this helps a bit.


Aedui & Aruernoi & Sweboz & Lugiones

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The events don't seem to be implemented yet (or at least I can't find them).


Baktria

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The tribute is bugged from what I hear, not sure why, maybe historic events can only fire once per game?


Hayasdan

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I think it gives you the ability to build the royal satrap in multiple provinces but I can't tell if it just allows one or if you will slowly gain more. It's also bugged because the Saka script was copied and it still refers to Saka for removing the point at faction turn end so I don't know if it'll work.


Numidia

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Pretty sure this refers to converting a nomad camp to a settled town/city.


Saka Rauka

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I gather you gain points on basis of your leaders authority and the size of your empire and you can spend points to build the highest level of government (royal domain). Same as Hayasdan I'm not clear on how many you will be able to build. It also deducts points for every royal domain and unlike Hayasdan it also deducts points for having the building in your cap which might not be intended and might also you over I can't tell either way.


Sauromatae

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I haven't checked but I assume it's a government form available in settled regions called a tributary based no the name of the counter.