Summer Miyu's Profile.
Noble Phantasm: Rampaging Obsession Mode.
>Miyu's attributes change to "evil" and she is constantly in a state of ecstasy.
>After collecting many fairies, a hole opens up that connects her to something otherworldly, and Miyu comes into contact with the concept of another dimension.
>Then Miyu realizes something.
>The world is full of Illyas.
>With only Illyas around, the world would be peaceful and happy.
>Miyu is going around using milk to turn everything into Illya.
>This liquid white substance is something named "Illya milk".

Lore wise, what is exactly "Illya's milk"?