The Astral Stair Map is found on the centre right of the map (Astral Stair). To get to it you must first go to the left of the merchant near spawn, 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. You will be in a room with the Map on an alter.
*This achievement is seemingly glitched on Windows at the moment. Loading it through cloud gaming seems to fix it.
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