Arenas are areas on the map where containers with special loot spawn. They are sourrounded by hills so you can’t leave the area easily if a player attacks you. There are 2 arenas on the map. One on the Grunni and one on the Norra island.
Info: This map was posted by the user @esme82 on the Longvinter Discord.

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Norra island arena

This shows the arena on the Norra island. It is located at the top of the island.