Dead Moon

The Dead Moon (also known as the Dark Moon) is an evil alien kingdom that is home to its ruler Queen Nehelenia and is where she formed the namesake carnival-like organization known as the Dead Moon Circus. The people of the Dead Moon, came to the planet Earth during a total eclipse of the Sun, and took the guise of the Dead Moon Circus.

History
The Dead Moon is a planet not far from the Silver Millenium and its civilization that is the reflection of that of the Silver Millenium much like Queen Nehelenia is the reflection image of Queen Serenity. Its people loved their ruler and worshipped her beauty and her talent for telling stories. One day, she came before an enchanted mirror and asked it to show that she will remain beautiful forever. It showed her a hideous old version of herself which shows to be her frightening and unfortunate future. She heard the echoing voice of a ghoulish figure that is the part of her future decay. She then drove to madness and developed powers, starts transforming her own people and subjects into the Remless and stealing their dreams. She learned she preserve her beauty as long as she steals other people's dreams which holds the energy nessecary to do so. She then formed the Dead Moon Circus and traveled to other worlds and steal the dreams of their inhabitants while the Dead Moon itself remains an empty and desolate civilization abandoned by the living ghosts of its own former people.

Queen Nehelenia was defeated and retreated to a mirror as she returns to the Dead Moon after the Dead Moon Circus was stopped by the Sailor Scouts.

The Sailor Scouts entered the world of the Dead Moon after Queen Nehelenia's subsequent awakening by an ominious force and her recent attacks on Earth. The evil queen's homeworld are full of vast and different landscapes. After her defeat, the Sailor Scouts made a choice to not destroy Queen Nehelenia, but to reform her with their combined powers because of her forgotten need for friends which was then eclipsed by her desire for eternal beauty. The energy that changed Queen Nehelenia back into an innocent child-like monarch that she was in her childhood, transformed all of the planet of the Dead Moon and it's people back into a glorious and peaceful world that it once was where Queen Nehelenia found friendship and love in her people who cared for her and her obsession with eternal beauty which became the essence of her mad and evil nature was gone forever.

Trivia
In the Sailor Moon manga, the date of their arrival was stated to be April 1.