On this page of Assassin's Creed Valhalla guide, you will find information on where to find the Hidden Ones Bureau (Assassins) in Lunden. This location is called Londinium Assassin Bureau, and searching it allows, among others, to obtain a mask from a Hidden Ones' set.
In order to find this bureau / hideout, you have to visit Lunden. It is a larger town, which is adjacent to the Oxenefordscire region in England. The hideout marker appears behind the eastern city walls.
Additional note - Before you investigate the hideout, acquire the Incendiary Powder Trap ability. It allows you to sacrifice the adrenaline to shoot explosive arrows. This ability can be most easily acquired by visiting the village of Walden in the Grantebridgescire region.
Go in a straight line south of the Hidden Ones' Bureau marker. You have to climb on a round wooden structure shown in the picture 1. This will allow you to discover a well. Jump inside to get to a huge cavern adjacent to the Assassin Bureau.
Don't try to open the front door because it is reinforced and barricaded. Instead, dive in the main cave. Find the boards shown in the pictures, which you can destroy with a white weapon, and enter the new tunnel.
Reach the very end of the tunnel. It is worth to dive again in order to find a smaller treasure - a large chest with nickel ingot, Destroy the pots lying in front of the chest to open it.
Climb onto the platform and start jumping towards the nearby bars.
After reaching the last bar, turn to the bureau side. There is a weakened wall shown in the picture. Use this Incendiary Powder Trap ability, and blow up the wall with an explosive arrow.
The most important treasure is waiting for you in the large chest - Hidden Ones' Mask which is one of the parts of the Hidden Ones' set. Note - If you want the mask to be visible during the game, after highlighting it in the inventory window, press the right analog.
There is also a Codex page in the hideout - it is one of the items desired by Hytham and related to his side quest.
Here you will find the Londinium Bureau Key, which allows you to open the door and return to the main cavern. In order to leave the hideout, reach the large boulder shown in the picture, and move it.