Complex sentence: After Mansa Musa's passing, it became the city of wealth and learning that we now know it to have been, that is, before it was destroyed by the hands of the Moroccans.
Split into simpler sentences: After Mansa Musa's passing, it became the city of wealth and learning that it is now known to have previously been. That is, before it was destroyed by the hands of the Moroccans.