On this page of Assassin's Creed Valhalla guide, you will find information on where to find the Hidden Ones Bureau (Assassins) in Jorvik. This location is called Eboracum 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 Jorvik. It is a large independent city, which is adjacent to the Northumbria region in England. Reach the cemetery marked in the picture, which is located southeast of Jorvik Theatre.
Explore the cemetery, and you will discover that the boards cover the hole. Destroy these boards by hitting them with a bow. Jump into the newly created hole to move to the underground.
You have to jump into the water. The underwater tunnel is very linear. Destroy the pots so you can go through the hole in the wall. You will reach a large, partially flooded chamber.
You can dive to reach the small treasure chest on the left or skip it and focus on going forward. Jump on beams protruding from water. Your destination is the hanging platform on the right.
After jumping on the platform, turn left and jump on the stable shelf. Destroy small objects and move one of the big ones to unlock a new passage. A straight path will lead you to the main room of the hideout.
Use Odin's sight to find the first key left on top of the cabinet.
The key can be used to open the gate leading to the crypt / vault.
The most important treasure is waiting for you in the large chest - Hidden Ones' Robes is one of the parts of the Hidden Ones' set.
There is also a Codex page in the hideout - it is one of the items desired by Hytham and related to his side quest.
There is also a second key in the same room. Take it and return with it to the previous room. You can use this key to open a large chest with an ingot.
There is an alternative exit from the hideout in the area - destroy the lock on the door to unlock it. The new path will lead you to a ladder and to another door with a lock. Get rid of the second blockade and return to the city.