Londinium Bureau AC Valhalla Wealth are the goods you'll get for seeking out the Hidden One's bureau. Here's where to find the bureau and what you'll get from it.
Londinium Bureau AC Valhalla Wealth: Where to Find
Londinium Bureau is one of the first of the Hidden One's bureau. You don't need any special abilities or gear besides the bow and arrow. There are also no story prerequisites to access the Londinium Bureau.
Head to the northeast of Lunden and head to the marker. You won't be able to get in right where the map says the bureau is, but you can climb over a large wall of wood nearby. Head to the hole and you'll come across pool of water. Dive in and go northwest until you have to break a wall to progress.
From here you'll have to do some platforming along the path until you get to a pole sticking out of the building. Shoot a flaming arrow through the hole in the wall at the urns to blast a way in. Head around to get into the Bureau.
Inside you'll get a Codex piece, a Hidden One's Mask and a piece of paper with some story about the bureau on it.