I think you guys missed a hint in the Celestalon's profile picture. To the left of creature there's two colored dots that differ from others. Their colors remind me of planets on art of Celestial Alignment.

What if one of the mysteries is to play celestial alignment + couple more cards in special order?

Harder on mobile - could be something related to flavour text, because you can't read flavour text in mobile client (but you still can get this info on the internet)