How would I get the sun element?

I'm stuck in the underground. I've collected all remaining elements (star, water, and forest), and now I'm trying to get that sun element. I've looked throughout all locations and found nothing.

Can you point me to the rooms where I could further investigate? I have included a map below.

Here is the map of the area. Do not click if you have not completed it or it may spoil it for you.

Solution 1:

Solve the puzzle in the room that looks like an egg:

How? (more images linked in spoilers below)

The creature wants a horn. The horn can be found in the one-eyed monster room (map), but you have to approach it from the bottom, via the infinite rooms.

Then what?

Once the lights are on, make your way to the top. (Don't forget the blue gem at the bottom, though!)