Nemesis was considered to be the tenth planet of the Solar System.
An illusionary planet of darkness where few things could grow. 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 impossible to calculate and no one was able to detect its existence for a long time.
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 was left alone and unchecked for centuries and it became a forbidden planet. The planet remained cloaked in darkness, but some time in the 30th Century it began to emit massive amounts of negative energy, indicating its presence. It was hijacked by the Black Moon.
At some point in time, the Death Phantom lost his physical body and became one with the planet.