The Unknown Arenas are hidden dungeons throughout Aurai, which can only be accessed once the player has completed a faction quest line for either The Blue Chamber Collective, The Heroic Kingdom of Levant, or The Holy Mission of Elatt. Each arena requires an offering to enter, or in some cases the player is required to have a specific status effect active on them.
Once you enter an Unknown Arena, you will be confronted by an exceptionally difficult boss. These are the most challenging fights in the game, and you will no doubt spend a lot of time dying to them. Even if you read this guide to learn about particular tactics, the fights will prove difficult. You will need to stock up on a lot of food and potions, and you will need to bring the best weapons and armour that you have available to the fight. The bosses have high health and high resistances, but each have their weaknesses. Use this guide to learn of those weaknesses so that you can prepare for each fight appropriately.
When each Unknown Arena boss is defeated, you will be able to loot 2 of 4 relics from an Ornate Chest. These relics can be used to craft unique weapons and armour when combined.