Random thought I had: As far as I'm aware the two requirements for breaking the Earthseal were 1: Destroy Heaven who you have lost every battle to so far and 2: Smuggle a Baby. It's like an endgame recipe that requires the eye of the hardest boss of the game and then some random weeds that you can get from the start.
The Otherworld Gate’s barrier was broken when Heaven fell, but because Aven instructed Celene to escape, she was able to put up a barrier with her powers as an Ancient. But it was incomplete and there was a small hole in it which a baby could be placed through. Stephanie’s presence on Earth slowly weakened the barrier until it shattered completely.