Crusader Kings III

If you go on any long term world conquest campaign, this will happen almost automatically. If you want to get this a little more quickly without conquering a huge empire, here are a few tips:

1) Where to start and what culture to be?

You'll want to start in an area of the map where there are many relatively small cultures. Ideally at least some of them share a common heritage and/or common language. Southwest Africa, India and Iberia are good locations for that. The latter is a good choice for going for multiple other achievements from the basic game and the fate of Iberia DLC at the same time. Iberia in 1066 already has 7 cultures on the Iberian peninsula alone that all share Iberian heritage and some of them also the Iberian vulgar language. An 8th one will very likely soon pop up when one of the several Baranis rulers in Andalusia decides to create an Andaluso-Baranis hybrid culture.

You should pick a culture that has"egalitarian" as its basic ethos, which gives you a massive 35% bonus on cultural acceptance progress.

Among the Iberian cultures, Basque and Asturleonese have this ethos. The easiest starting character to do this in Iberia is Alfonso Jiminez of Leon, if you have him convert to the local Asturleonese culture. He has a very high intrigue skill and can thus enlarge his realm quickly by inheriting Galicia and Castilia when his siblings die in mysterious tragic accidents. If you want to go for Southwest Africa, the highlighted character Daurama Daura is a good choice, since the Haussa culture is egalitarian and you can go for her scenario achievement to unite Africa.

2) How to increase and maintain cultural acceptance.

The cultural acceptance of each culture is subject to constant change. While certain events cause instant positive of negative changes (like e.g. declaring war on a ruler of that culture or giving a title to a vassal of that culture), some factors make the acceptance increase constantly over time. They are mainly countered by the factor "decay towards baseline", which decreases cultural acceptance and becomes larger the more the current cultural acceptance is above their acceptance baseline. Since your steward can only promote acceptance for one culture at a time, your goal must be to increase the factors that cause acceptance growth so much that they are bigger than the maximum decay towards baseline factor at 100%. Raising the acceptance baseline also helps with this, because the decay factor grows with the distance to the baseline, so the decay factor at 100% will be bigger if the baseline is 0% than if the baseline is 15% or 30%.

Factors that improve the acceptance baseline:

- cultures share the same majority religion (like e.g. Christianity, Islam, Hinduism etc)
- cultures share the same majority faith (like e.g. Catholicism, Ibadism, Shivaism etc.)
- cultures share the same heritage (e.g. Iberian, Frankish, Central African, Arabic etc.)
- cultures share the same language
- cultures share the same ethos while being geographically close

You can mostly only influence the first two of those, by conquering the majority of the other culture's counties and converting them to your faith.

You might also be able to fulfill the missing "geographically close" requirement with a faraway culture of the same ethos if you convert counties near them to your culture, but that likely won't even become relevant if you play in the suggested regions, since "geographically close" is handled quite generously and you have to spread very far to encounter cultures in your realm which aren't considered geographically close, it's just there to prevent e.g. the Norse and the Mongols feeling mutual attraction just because they are both bellicose, even though they come from opposite ends of the globe.)

You can do something about the common language if you create a hybrid culture and pick the language of your new culture strategically. E.g. if you play in Iberia and expand into North Africa, you could create a hybrid culture with the Butr or the Bahranis and pick the Berber language, so you now share the language with those two cultures while still being tied to your fellow Iberians by the Iberian heritage (even though you'd of course lose the common language with them). In theory, you could also change your cultures ethos during hybrid culture creation to match that of other cultures, but it's better to keep the egalitarian ethos, the 35% bonus on acceptance growth by far outweighs the improvement of acceptance baseline that you could get from picking the same ethos as another culture.

Factors that improve acceptance gain overtime:

- intermingling in realm
this is caused by counties of both cultures being in the same realm. The number is variable, it depends on how many counties of both cultures exist in your realm and how many counties of the foreign culture exist outside your realm. A culture that is completely or almost completely absorbed into your realm will grow in acceptance much more quickly than a culture with the most of their counties outside of your realm. To get high acceptance, you should try to conquer them fully, but mostly refrain from converting counties to your own culture, since this will cause both an instant drop in acceptance and diminish the intermingling factor.

- vassallage
This number increases depending on how many nobles of that culture are your vassals or vassals of other nobles of your culture and vice versa. It is especially helpful if the head of the other culture is your vassal. So if you absorb a culture into your realm, always make sure that you grant at least one of the conquered counties to a lord of the local culture.

- bordering your culture
This is true if any county of that culture directly borders a county of your culture, even if that county is just an isolated enclave of your culture that is far away from your cultures original homeland and has just been recently converted. Thus it can be helpful for cultural acceptance to convert few strategically selected counties to your culture to create the bordering condition with other cultures.
E.g. let's assume you play the Asturleonese and have succesfully expanded your realm into the north-east of Iberia. Your culture already borders the Galicians, Castillians and Andaluse, but none of the others. Now you have your steward convert just two counties, Saragossa and then Rosello and voila: You now also border the Basque, Aragonese, Catalan and Occitan cultures.

As already mentioned, egalitarian ethos gets you a 35% increase on all of that acceptance gain.

You can get a further 25% of all acceptance gains by having or establishing the xenophilic tradition in your culture (by clicking reform culture and investing fame to establish xenophilic as a new tradition or by creating a hybrid culture with a xenophilic culture).

To push the acceptance, it is also helpful to do lots of traveling within your realm and always take a cultural ambassador with you when you do, which will instantly raise the cultural acceptance by 1% with every local culture you encounter on your travels. Doesn't sound much, but it adds up if you travel often. Travelling costs you almost nothing if you visit the events of other lords (e.g. hunts or feasts) and you can always take some little detours on the way back to touch some more of your vassal cultures. There will also be random events especially when you hold court that give you a chance for instant improvements of cultural acceptance.