On this page of Assassin's Creed Valhalla guide, you will find information on where to find the Hidden Ones Bureau (Assassins) in Ledecestre. This location is called Ratae Assassin Bureau, and searching it allows, among others, to obtain gloves from a Hidden Ones' set.
In order to find this bureau, you have to visit Ledecestre located in the Ledecestrescire region of England. The bureau marker may not appear until you climb the ruins. You will also notice the great painted sign of the Order of Assassins.
Look around near the marker, behind the hanging cargo shown in picture 1. Hit the red element to drop the cargo and make a hole in the ground, into which Eivor can jump.
Passing through the first corridors of the hideout is not complicated. You have to slip through a few holes by the ground.
The most important treasure awaits you in the main room of the abandoned bureau - it's the chest with Hidden Ones' Gloves. This is one part of the armour from 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. Return to the surface in the same way.