Barbarians are marginalized peoples living off the edges of the map in isolated and inhospitable mountains, forests, or deserts that will occasionally rise up to pillage, settle, or even conquer their more civilized neighbours. They can present a significant threat to all states in the vicinity of their strongholds and will often require a dedicated military response to prevent widespread pillaging and destruction, but are also the source of pops and potential subjects that a state can use to make itself stronger.
Barbarian Strongholds are wasteland territories from where barbarian uprisings come. These can be identified as the brown-to-red territories highlighted in the Barbarian Power mapmode. Each barbarian stronghold has has a modifier, such as Barbarian Stronghold or Major Barbarian Activity, which provides a ticking monthly modifier to the stronghold's Barbarian Power. Barbarian power reduces the civilization level of the province and increases the monthly chance that a Barbarian Uprising will occur.
Barbarian Strongholds cannot be directly interacted with, but they can slowly be civilized by using the Civilization Effort Governor Policy in a neighbouring province. If the province's civilization level is high enough, there will be a monthly chance of increasing the barbarian stronghold's civilization level, which decreases barbarian power growth. Once barbarian power growth becomes negative and barbarian power goes down to 0, the barbarian power modifier will be removed and the province will be permanently pacified.
A barbarian stronghold with positive barbarian power has a chance of producing a barbarian uprising each month, based on its total barbarian spawn chance modifier (+0.20% per barbarian power). The uprising will create a new barbarian horde that gets 1 cohort for every 2 Tribesmen pops and 1 for every 10 total population in all the territories adjacent to the wasteland stronghold territory, each rounded up, and generally following the levy composition of their culture. The barbarian horde will spawn in the stronghold and then move into a random neighboring territory and begin sieging it. Hordes will take the culture and religion of the wasteland region they spawn from, which is set at the beginning of the game and cannot be changed, have a commander that leads their horde, and will take a name from the barbarian name list of their culture group.
Barbarian hordes are hostile to all factions and ignore any borders, slowly reinforcing any losses at all times as long as they are not sieging any territories. Any territory that comes under the control of a barbarian army that is not in the zone of control of a friendly fort will lose civilization level based on its current civilization value and the size of the horde, according to the following formula:
Territories with at least 50 civilization value will not lose any civilization value at all. This means that barbarian hordes are generally the most destructive in areas that already have relatively low civilization value, such as in tribal areas (especially settled tribes, as migratory tribes generally have little use for civilization value). Every occupied territory will also gain 1 Tribesman pop of the horde's culture and religion and disband one of the army's cohorts, representing part of the horde settling down.
Hordes will continue to move around, rampage, and occupy territory as long as they exist, and can wander quite far from their original strongholds and cause considerable destruction to a region if the states there are unable to defeat it. Destroying barbarian armies will give the victorious state the plunder that they have accumulated through looting provinces, may yield 1 slave pop for every 10 cohorts in the army that can be resettled in nearby territories, and will return any territories that they still occupy back to their owners. Forts near a barbarian stronghold or near borders where they are expected to come through can be quite useful as they protect neighboring territories from losing civilization value and can stall the advance of a horde until an army arrives.
If all of a country's territories are occupied by barbarian hordes, the horde occupying the capital will take over the country. The horde's leader will become the state's new ruler and found a new major family (displacing one of the old ones), the country will adopt the culture and religion of the horde, the state's government form will be converted into a migratory tribe, and the horde's army will be transferred to the country. This is the only way to convert from a republic or monarchy into a tribe, and the only general way for a country's culture to change.
If the barbarians are too powerful to fight off or the state seeks to take advantage of their strength and population, a barbarian horde can be negotiated with through the diplomacy screen. This screen displays the number of occupied territories, accumulated plunder, possible slaves, total army size of the horde, religion, and culture of the horde and offers several possible diplomatic actions:
- Pay Off - Always available, allows the country to select an amount of gold to offer to the horde. If the horde accepts, they will cease being hostile to your troops and provinces and will go off to raid another country. A barbarian horde will require more gold to pay off if the country's armies are significantly weaker than the horde, and if the horde is occupying a significant amount of territory. Only one country can pay off a horde at a time; if the same horde gets paid off by another country, they will become hostile to any countries that paid them off previously.
- Settle - Only available if the horde controls at least one of the state's territories. If the horde accepts, this will disband the horde and create 1 Tribesman pop of the horde's culture and religion for every 4 cohorts in the horde in one of its occupied provinces. They are unlikely to accept this if they are stronger than the state making the offer.
- Create Tribal Vassal - Only available if the horde controls at least one of the state's territories. If the horde accepts, all their occupied territories will be released as a tribal vassal of the state with the horde's culture and religion and the migratory tribe government form that will inherit the horde's armies, leader, and name. Hordes will generally be quite likely to accept this.
- Demand Surrender - Only available if the horde controls at least one of the state's territories. If the horde accepts, this will remove the horde, return all occupied provinces, and give the state gold and slaves as though the army was destroyed normally. A barbarian horde is very unlikely to accept this unless the state is significantly stronger and has recently defeated the horde in a battle.