Probably been said already but definitely the line from The Watcher that explained that Hydaelyn sundered Zodiark, the star, and herself. It's not even a typo because all languages across the board say that Hydaelyn sundered herself. So why did Source Hydaelyn need to send Minfilia to the First to stop the flood? Shouldn't the First's reflection of Hydaelyn as a Primal of divine light be able to mitigate or even absorb the flood into herself? It's light aether after all and she's a light based primal.