Method

Follow the route to Demona and go up into the main (pentagrammatic) room. Make your way to the northeastern-most staircase while fending off an Elf Arcanist and some Warlocks and go down. You will come to a long hallway you'll have to follow all the way until you get to a big room filled with Elf Arcanists and Warlocks. Go up either staircase in this room and you will encounter more Warlocks upstairs. There are two chests in the south of this floor, with the left one containing a Stealth Ring and the right one an Energy Ring (they are marked purple in the image below).