Disclaimer: Please beware that this page contains spoilers for Divinity 2 Original Sin.
If you have just recently come out victorious in the Driftwood Arena in Divinity 2, chances are that you have looted all the items in the arena vault. Among the items you can find there, one is a mysterious Key of the One. You might be looking for a door somewhere in the arena to open it, but you will not be successful. The key opens a door that is in a completely different area.
Finding the door that the Key of the One opens
The door for the mysterious key lies on the Nameless Island. To be more precise, you have to enter the Academy on said island.
As you progress through the Academy per the normal main quest of Divinity 2, you will find a locked door in the Teacher’s hall. The door leads into a room called “Armaments of the One” and can be unlocked with the Key of the One (it can also be lockpicked).
The room contains a single, big portal which will lead you to the Proving Grounds. Enter the portal and you will find a dead bird that you can talk to with Spirit Vision. The phantom bird named Ghechswoll the Arena Master will inquire about any other arenas in which you have been successful before. Ultimately, she will offer you an even greater challenge for you to overcome. Accept it and enter the big chamber behind the Arena Master.
The Great Guardian
The Great Guardian is a huge automaton and the boss of the proving grounds and this iteration of an Arena. Upon entering the room it will immediately talk to you and order you to leave. Stepping closer or choosing the proper dialogue option will initiate combat.
Combat will also reveal 2 Eternal Protectors and 2 Eternal Sentinels. You can simply focus the Great Guardian, as his minions will fall dead the moment you defeat him. Rest easy though, as you will gain experience for all the enemies in this fight. The Great Guardian himself offers a mighty 97,560 EXP, while his minions all give 27,900 EXP each.
The fight alone should not be much of an issue. Despite the fact that the Guardian is huge and scary, its size is actually a big disadvantage. Many AOE source spells will do devastating damage against it, as the body size means that it will eat all the damage. Simply cast a huge spell like Thunderstorm or Hailstorm.
After the fight, you can loot the Great Guardian for the “Blade of the Swormbreaker”, which is one of the parts that you need in order to craft the Swornbreaker weapon. The Guardian will also drop high-quality Divine-level loot.
Once you return back to Ghechswoll the Arena Master, she will now solemnly proclaim you as “the One”.
Even though you can pick the lock of the door that leads to the Proving Grounds, you should still have the Key of the One as overcoming the Driftwood Arena comes far before the Academy.
This whole area is completely optional, but as you can see, it will grant you lots of EXP and high-level loot. The piece necessary for the Swornbreaker weapon is also pretty important.
If you are looking for more parts of the Swornbreaker, refer to this article: