Assuming the Fatui are the cause of his constellation now falling to the ground, Mona works out that the biggest rock, which should be harboring Leonard’s spirit, would have fallen atop the mountain he never managed to conquer.
Most of the words have faded to the point where we have to question why it’s freely available in the library.
Using her magic, she’ll deduce that Leonard died around 2,000 years ago and was never able to conquer the mountain then known as Pilos Peak.
Take it back to Mona to see if she can work it out.