Who doesn't love it when a wandering troll from the black forest starts beating up swamp creatures left and right? I appreciate the fact that undead types attack everything and the forest types won't attack their own. In my head, undead types are mindless killing machines and the forest types are protecting the environment they call home.
What if by default, certain creatures wouldn't attack you unless provoked. Then by actions of the player, good or bad influence would determine an alliance or a declaration of war.
Examples:
Cutting down a tree = -1 forest action Planting a tree = +1 forest action
Killing a deer, forest creature, building a home in the black forest -5
Killing an undead +5
at -100 you are KOS by all forest types at +500 you get access to special items for unique decor or maybe armor and weapons. At -500 you start to get raided, but after the raid you are +200
I'll roll a +1000 swamp "slime lord" please
Source: https://www.reddit.com/r/valheim/comments/w5cfuy/valheim_factions_or_gangs/