| Okay, here's a description of where the chest was for me.
Taking the direction of the room with the four water snakes to be south from Belsornig's main chamber, there is an alcove on the south wall of that chamber, right by where the juggernaut starts out. There are two doors on the east and west sides of that alcove. Take the door to the west (towards the bottom of the screen), and you'll be in a room with another door, this one heading south. That door leads to a long room (probably the "narrow" room to which you refer). The chest should be against the east wall of that room. |