The Sun is the level you find when you reach the bottom of the stairway on The Descent. It is a level packed with infinite Light, perfect if you need to harvest light.
Actually, harvesting light is the only thing to do on The Sun, so do not come here if you do not have enough harvesting fingers or want to harvest light.
If you reach the cliffs and start going down (dash in safe surfaces because it is easy to fall) you will find a hidden Light Pit on a little cave.