Hidden Ones’ Robe
This armor piece is found in Jorvik.
Travel to the city and go to the southwestern section.
Next to some graves, you’ll spot a breakable wooden panel that lets you drop down below.
In the large chamber, move the blocks so you can squeeze through an opening.
You should see a note telling you to “seek high.” Look above the shelf to find a key. This will unlock the main room where you’ll find yet another key (for the ingot chest).
Anyway, grab the chest piece and the codex page.