“Somewhere a Great Rune has broken” in Shadow of the Erdtree explained

What does “Somewhere a Great Rune has broken” in Shadow of the Erdtree mean? The message comes out of nowhere, followed by another that says “and so too has a powerful charm”, but what are you supposed to do with that? If you’re like me playing Shadow of the Erdtree, you run to a Site of Grace ASAP to check all your existing Great Runes and make sure none of them have been lost, but that’s…