On this page of Assassin's Creed Valhalla guide, you will find information on where to find the Hidden Ones Bureau (Assassins) in Glowecestrescire region. This location is called Temple of Ceres Assassin Bureau, and searching it allows, among others, to obtain leggings from a Hidden Ones' set.
In order to find this bureau / hideout, you must visit Temple of Ceres. It is located in the forest, at the western end of the Glowecestrescire region and to the west of the Glowecestre town. The temple is very impressive and cannot be missed.
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.
Make sure you have at least 2 adrenaline slots because you will have to use at least 2 explosive shots.
The entrance to the dungeon isn't hidden in this case. Help yourself with a torch to illuminate the area. Find a group of small objects that can be smashed and reveal the hole hidden behind them.
Soon, you will reach a location where the poisonous gas is floating. Contact with fumes will lead to a gradual loss of health points. Fortunately, you can easily neutralize this problem - throw a torch at the green fumes, and the gas will explode and disappear for a while.
In the next room, use a bow to destroy the boards on the floor. This will allow you to jump to a lower chamber.
Get rid of toxic fumes again. Find the first weakened wall there and use explosive arrows to demolish it - the Incendiary Powder Trap ability. This way you will reveal a large chest with an ingot
This chamber has also a different exit. Get to it and again get rid of the poisonous gas. You must destroy the other weakened wall. Use an explosive arrow as before.
You will find yourself in the main chamber of the crypt. The biggest treasure is waiting for you in the big chest - it is the Hidden Ones' Leggings, which is one of the armor parts from the Hiden 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.
In the main room in the hideout, there is also an alternative exit from the dungeon. Locate the door with a blockade and destroy the lock with a white weapon.