The Arena of the One is a location in Divinity: Original Sin 2
Noosey tells you about this arena.
- If you have Beast in your party, talk to Carson, the Snickering One for a Companion Origin Moment and you will receive a Magister's Key.
- There are some chests (including an Ornate Chest and other containers you can loot in the fight area of the arena.
- You can only loot those while in that area. You cannot teleport to that area in any means.
- Loot those chests while in combat, since once you defeat all the enemies you will automatically be teleported back to the starting area of the arena and won't be able to loot those chests anymore.