Players by themselves dont change anything besides a small tune to monster stats withing 100m to adjust the difficulty

The world events are tied to things that actually happened in given world, thats it. Any items from "outside" won't change a thing, except maybe if you brought a boss head and placed it on the stone, for that im not sure.

I dont think zerg fits Valheim either, but tbh thats only the seekers, rest could work. And frankly actual zerglings are much different... the wings and poor combat ability are sequels idea. Proper zergling would murder everyting in sight.