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This is not a spoiler, I will not tell you the answer. There’s no way in the game to see this after you hear it once and other sources on the internet seem to just skip it and give you the answer. So, here’s just the riddle, yes the riddle you were looking for 😉
Down the stairs… watch your steps, don’t fall apart or it’s your end
Round a corner, turn around, there’s.. your way, in front of you
All that’s closed can be… opened too, if you see… beyond …its looks
If you immediately after that cutscene start looking around every stair you find, you’ll soon find a sign with that riddle. But you need to pay attention to your surroundings, because it’s easy to miss.