Factions
There are six main factions in the game:
Besides the main factions, there are secondary factions:
NPCs of the same faction won't fight each other but will attack members of other factions unless they are at peace.
There are also two special factions: Gods and Demigods. Gods will not fight anyone except as part of the main story. Demigods are usually not aggressive by default, but you can eventually fight most of them if you choose.
Animals and beasts are unique in that they will attack other types within their faction but not their own kind. For example, a wolf may fight a boar but not other wolves. A werewolf may fight a vampire but not other werewolves.
Faction Wars
After completing the wars portion of the base game's main quest or defeating the Seraphim in the alternative main quest, you will be invited to join a faction on Koren Island.
You can support only one of the six main factions. Joining a faction allows you to assist them in conquering the world under their banner through a series of siege wars.
You can always enable faction war borders on the map to check which faction is controlling each area.
Faction Maps
Each faction triggers different events and has unique mechanics and NPCs. This faction war questline can be slightly altered depending on the outcomes of sieges and decisions made in the base game's main quest.