The Chaos Rosalia is found on the far top right of the map (Astral Stair). To get to it you must first go to the very top of the long corridor, then enter a hidden "heart shaped" door disguised in the wall. You will be teleported across to a new room, simply go to the right of this room and enter the next door. Do this twice and you will be in a room with the Chaos Rosalia on an alter.
Visual Guide: