From the aforementioned walkway, drop down to the east into the blood pool. Strictly go north and shoot the barrels. Walk through the fake wall right of the barred blood fall. Lower the lift and ride it up to access the balcony (sector 55) with a blue armor on it.
Behind the yellow door, in the south of the red skull key, east of the computer map and west of the plasma gun, push the slightly misaligned part of the wall to lower a lift. Ride it up and follow the walkway. There is a hideout (sector 100) containing a soul sphere at the top of the curved stairs.