The Trisection is found on the far bottom right of the map (Astral Stair). To get to it you must first go to the very bottom 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 Trisection on an alter.
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