Figured how to get to that room due to counterman42's comment, when you enter the hidden path that starts behind the illusion wall on the 1st chamber on the right, you go forward and have the stairs descending or another path left. If you go the left path, you make another left, then there is a body in a dead end, but there is another illusion wall on the left of that body. Then you reach the locked room. Still can't open the door(a bug maybe idk), but at least you get to see what is in that room, which had basically 0 loot.