Hidden Ones’ Leggings
This armor piece can be found in Gloucestrescire.
Head over to the southwestern edge of the map.
You’ll see a shrine that you can enter.
The interior has several explosive barrels that you can use to blow open some parts of the cavern.
However, be forewarned that there’s lots of poisonous gas. Although there are herbs that you can eat, make sure you still have enough rations.
Eventually, you’ll reach the Hidden Ones’ Bureau which has the leggings and codex page.