A theory about Ashe's bow and Swains interaction

Since its hinted that the bow MIGHT not be the bow of Avarosa and I believe there is a simple explanation to it. When the watchers came Lissandra sacrificed her sisters and the Iceborn army. But she wanted no one to know about what she had done. So she created a grave where Avarosa rests and created a bow identical to the original one. So it could add up to why she suddenly vanished and in the same time covering what she really did. And that would make sense with Swain and Ashe. She thinks its the bow of the legendary Avarosa because she found it there where she is "buried" but in reality its a copy of the original one.