Me and my brother just found out how to do it and get the royal guard talisman.
On the side with the lane of lava there is a lever vertical right beside that sentinel if you push that lever the left side gate unlocks and goes down then you can hit the lever on that side to open the gate to the chest which contains the royal guard amulet.
i had put perception on my brothers character im not sure if you need it to initially see the first switch but i hope this helps for you all to get the chest.
