Quote Originally Posted by TankHunter678 View Post
The raid storyline makes it clear that if they stayed in the Empty for too long they would turn into Sin Eaters. The Empty was created by Eden passing through and passively performing its function of transitioning all aether in its vicinity to extreme astral polarity. Minfilia stopped the Flood by asserting control over it with the help of the First WoL souls and putting it to sleep.
On the off chance that this wasnt a typo, Eden turned all aether in its vicinity to the umbral polarity in order to cause the flood.