Driftwood Arena Treasure in Hidden Hatch - Divinity: Orignal Sin 2

Once you beat Murga in the Arena, you will get a key. On my first playthrough I missed this. The key allows you to open up a hatch, which contains "Key of the One" (used later in the game) and some sweet loot.

The hatch is at the left side of the arena:

Inside you will be closed of by a petrified door. You can either cast Bless or destroy them with physical damage. There will be two doors. Otherwise they won't budge:

After the first doors on the left side, on the platform with "Pot still" there will be a valve which you need to turn to disable the steam:

Then you are free to go to the Treasure room and take your sweet loot.