Astral Ruins

The Astral Ruins is a serene area accessible from the large pipe in the Grassy Pipes, after switching the orb from purple to yellow.

Features
This area features a temple with moon-shaped windows and potted flowers everywhere, as well as moon-shaped characters walking around who speaking in Wingdings when interacted with.

There are several rooms that can be found here, one leading to a winding narrow path lined with hands floating like butterflies and large statues, similar to the one seen outside of the Concrete Tree. Reaching the end of this path will bring you to the Sun Lord's domain, where you can find the Sun Lord sitting on a throne, being fanned by two moons. The Sun Lord reacts to several effects. Equipping the Sunflower will cause him to look at you, and the Nightcap will make him laugh at you. Attempting to attack him with the Lily of the Valley effect will result in him grabbing Totsutsuki and taking her offscreen, presumably killing her. This will cause her to wake up.

Directions
Totsutsuki's Nexus → Lightbulb World → Grassy Pipes → Astral Ruins

Trivia

 * This is the first area in the game to feature an event that wakes up the player.