Nemesis

Nemesis (ネメシス) was the tenth planet of the Solar System, and the base of the Black Moon Clan in Sailor Moon R. It was a dark place where very few flowers grew. The planet itself was made of Evil Black Crystal, which was capable of absorbing light, much like a black hole. This would cause the planet to appear and disappear from the sensors tracking it in Crystal Tokyo, making its orbit seem erratic and difficult to trace. For a long time, the planet remained cloaked in darkness, but some time in the 30th Century it began to emit large amounts of negative energy, indicating its presence.

Soon after the creation of Crystal Tokyo, there was a challenge to the peace made by the Death Phantom. Because he was human, Neo-Queen Serenity did not want to kill him, so she banished him to the planet Nemesis and it became forbidden. Many years later, there was another revolt on the Earth. After the battle, those who did not want to stay left Earth and ended up on Nemesis where they came under the influence of the Death Phantom.

At some point in time, the Death Phantom lost his physical body and his will became one with the planet.

Trivia

 * Do to the fact that Pluto is still considered a planet in the series, Nemesis was called the tenth planet.