How do I get it? It's when you enter the south gate there's a room with people walking about and it's up on a roof. Room with cage doors and a box you can jump on but doesn't get you high enough and there's no ladder up there? Also what's in it?
It's when you teleport to the South Gate of the ruins and walk straight into the ruins after unlocking that area with a pilgrims crux I think it looks like cage doors maybe like prison cells and above you, you can see a floating treasure sphere but you can't jump to collect it looks like it's on a roof.
If it's in the room with the massive caged area that has no doors, you get into it from below. Carry on the the left of the cage and go down every tunnel you see. Eventually you should get to a room with a ladder gaurded by an earth eater. Defeat and drop down. Follow the rooms to a ladder leading up. It leads to that caged room and there's boxes inside you can jump on to get that treasure sphere. One other in there holds the golden scarab needed for a Canvas of Prayers quest.