How Much Is Known About That Dagger?

The fact that Alison knows it all is not great. She doesn't know what it means, clearly. But you're absolutely right in the fact that there's two interesting things here. One, she now knows has a piece of this prophecy that is only supposed to stay with the ruler and given to the heir. And then the second thing about this is we don't know how this was passed from ruler to heir. How did it get to Rhaegar if it indeed did? How did it even get to Viserys? Because the king before him, Jhaerys, was preceded by Magor who usurped the throne from the rightful heir.