In the manga, she is the dark mirror image of Queen Serenity; the self-styled Queen of the dark center of the moon who first made herself known during the Silver Millennium. Queen Nehellenia arrived at the celebration of Princess Serenity's birth. Swirling with dark energy, she confronted the Queen, who used her wand, the Moon Stick, to seal Nehellenia inside a dark mirror world forever. As retaliation, Nehellenia cast a curse that the Moon Kingdom would fall and the princess would never inherit her throne. Her dark magic ultimately brought about the end of the Silver Millennium, but she lived on. However, over one thousand years later, a solar eclipse allowed her to spread her curse to the earth by way of infectious nightmares aided by her minions, the Dead Moon Circus. Nehellenia still believed that she was the rightful owner and heir of the illusory Golden Crystal; and set out to steal it.
In the anime, Nehellenia is the dark mirror of the moon who had the dream to stay young and beautiful forever. Her Kingdom was located on what she called a lonely and cold asteroid. She understood that she would never be able to attain her dream when she looked into an enchanted mirror she saw herself aged and ugly. In order to retain her beauty, she consumed the dreams of the people of her kingdom. Those subjects became Lemures, the main monsters of the day in the series, and formed the Dead Moon Circus.