On this page of Assassin's Creed Valhalla guide, you will find information on where to find the Hidden Ones Bureau (Assassins) in Wincestre region. This is a location called Venta Belgarum Assassin Bureau, and its search allows, among others, to obtain a unique assassin's dagger.
In order to find this bureau / hideout, you have to reach the large urban region of Wincestre. Investigate the western part of the city, specifically the area south of St. Peter's Church (one of the viewpoints).
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.
The entrance to the dungeon is a large hole shown in the picture above. You have to jump into it.
You will find yourself in a partially flooded room. The entrance to the Hidden Ones bureau is barricaded - you have to look for another way. Jump into the water and dive. Find a long underwater tunnel and swim through it.
When you get out of the water, start lighting your way with a torch. You have to find the weakened wall to demolish, shown in the picture. You can do this by hitting it with an explosive arrow, i.e. by using the Incendiary Powder Trap ability.
The newly unlocked passage will take you to the main room in the crypt. The most important treasure is waiting for you in the large chest - this is the unique dagger of the assassins - Suttungr's Claw. Its unique feature is increasing critical damage after each successful attack.
There is also a Codex page in the hideout - it is one of the items desired by Hytham and related to his side quest.
You must return to the first chamber. Start by moving a larger object and then the smaller one.
To leave the crypt, you have to climb up and get out with the same hole you jumped into at the very beginning.