Accessing the Hidden Ones Bureau in Colcestre, Essexe. This gets you the next Codex Page and The Hidden Ones Mask.
Grab an oil jar and run across the tree branch to expose the entrance. Once you get to the submerged area, use the passage on the upper left to get to an area where you can shoot the barred door out of the main chamber. From there, go back around to the upper right path to enter the room and collect the loot. There's also an Armory Key here which opens up a door to a Nickel Ingot chest
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