Skeletons and draugrs cooperate, but skeletons and greydwarfs fight. I assume draugrs and greydwarfs would fight. So there are divisions and alliances among the mobs, suggesting some kind of history there.

I doubt the devs have decided on a backstory for Valheim, or if they will ever do more than throw out hints, which is OK with me. Not every mystery needs to be solved.

On the other hand, they may go in for an elaborate backstory (preferably without answering every question). It's cheaply done, it just has to be on runestones and the occasional ruin with a story to tell.