



Runeard magic theory
Runeard attacked the Northuldra because he was fearful of magic. However, the secret room in the castle implies someone in Arendelle used to know a lot about magic.
This could mean a lot of things, but my theory is someone in the past tried to study magic to take over the throne. Maybe it's even the guy in the cat portrait, that's why his painting hides the entrance to the room.
Another theory is Runeard himself knew about the magic and wanted to keep it for himself, but I think that's less likely.