Celestials are beings that are no longer able to visit Revaliir. They are responsible for the creation of not only the World Trees but also the world itself. It's said that one Celestial in particular wanted a child so large that it would rival what would become the Sun. They are more powerful than the Gods and the Trees but have little to no interest in the planet itself, or so many think. They learned long ago that the very mortals they created would be their downfall. As such, they became the thing of legends.
My sister the stars do sing. They write a song that only a few can sing. We've lost touch with our roots and someday that is going to be our downfall. Look at the Gods and how quickly they forget their roots -A Muse at the Tower of Edhit