There were so many hints, really. Celestia and Luna talk about it, then at the end of the ep, WHAM



That book is the same one used to turn Twilight into a princess.

Just thought it was kinda cool :3 Gonna rewatch Twi-corn ep now. It's so rushed that I'm not that excited about rewatching it, but I love me some Twilight Princess (and I mean that in both regards)