On this page of Assassin's Creed Valhalla guide, you will find information on where to find the Hidden Ones Bureau (Assassins) in Colcestre. This location is called Camulodunum Assassin Bureau, and searching it allows, among others, to obtain a Hood from a Hidden Ones' set.
In order to find this bureau / hideout, you have to visit Colcestre. It is located in the Essexe region of England. You must reach the southern part of the city - we marked it on the picture. There is an icon of a dungeon and two yellow icons of wealth.
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.
Locate the knocked down statue and start climbing to it. This will allow you to approach the secret entrance to the dungeon.
Your task is to use an explosive arrow on a weakened fragment of the floor - it is shown in the attached picture. This fragment will explode and you will be able to enter the underground.
Light a torch to find the path more easily while exploring the dungeon. Follow a straight path at first, but stop after noticing the skeleton. Next to it there is a place to climb.
Go ahead. On the way, Eivor must move the interactive object to expose the hole. Ultimately, you will reach a large, partially flooded chamber.
Go straight. The main entrance to the hideout is blocked. Turn right and climb to the top balcony.
After finding yourself on the balcony, rotate 180 degrees. You must jump to the opposite balcony shown in the picture.
Jump down and you will find yourself next to a closed door. Dive right next to them. At the bottom you will find the first key. Take it and return to open the door.
In the new area, position yourself at the bars. You have to aim a bow at the barricade on a door visible in the distance and destroy it.
Now reach the newly unlocked door. This means that it's necessary to return to the main flooded room. Reach the right balcony again. Jump down and go to the door leading to the main room of the assassin's bureau.
The most important treasure is waiting for you in the large chest - Hidden Ones' Hood 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.
Take the second key from the main room of the hideout and return to the flooded chamber. Dive and find a side underwater tunnel. It will lead you to the barracks door shown in the picture.
In the barracks, you will find a large chest with an ingot. Use the stairs to reach the door with the barricade. Destroy it to unlock the passage.
In the next room move the heavy cargo. All you have to do is reach the exit from the crypt.