This page of the Assassin's Creed Valhalla guide will show you how to get the gear hidden in Venonis. This is one of the more problematic collectibles because the ruins with the secret are closed.
The secret is located in Venonis in the Ledecestrescire region. It is a huge ruined building – you can't get to it at first because the main entrance is closed and barricaded.
Start by climbing up and reaching the roof of the large building. Go to its back. In the place from the picture, you can walk on the line to the balcony on the tree.
Turn right and jump to slide down the rope to the lower balcony.
Now turn to the building with the secret. You must use a bow and shoot through a hole in the wall – you have to hit the lock on the front door.
Destroying the blockade will allow you to use the main entrance to the building. Deal with the opponents or sneak past them.
The chest with the secret is hidden on the first floor, behind the boards shown in the picture – destroy them. The chest has Hrafn Guard light shield.