The Hallow Sun and the Corrupt Moon actually play a role in Lunar Soul: that Tower room that connects to the Lunar Soul area? It holds the key to outright changing which direction the sun/moon is shining at. The sun and moon will basically switch places.

Those aren't real suns or moons, though; those are mechanical and magical at the same time.

The final boss can only be battled in moonlight, otherwise he appears dead. However, his sister can only be battled in moonlight as well, otherwise her room is sealed off.

Reaching the final boss in sunlight triggers the bad ending.
Reaching him after defeating the witches before beating his sister goes to the good ending.
Reaching him after defeating his sister first, then the witches, leads to the best ending.

Or so I planned... ^^;