Naströnd collects/ captures neutral enemies
It seems that neutral enemies like draugr, wolf and valkyrie can wander into the naströnd tile from other neutral tiles but can't leave. And there seems no limit how many enemies can be packed into the naströnd tile. They just stay there and can't leave until naströnd is cleared, then they behave like normal neutral enemies again.
It happened while I was playing the valkyrie galore mission (conquest) but I also saw a picture on the northgard discord channel of someone who had 36 draugr in a naströnd tile during a normal multiplayer game.