Enter the Silver Millenium. The beginning of Sailormoon - the roots of the senshi, the princess and the prince. What once was bliss, crumbled and faded away....but they lived on.
Chiba Mamoru had lived as Prince Endymion in his past life. While Queen and Princess Serenity lived on the moon in Silver Millenium, Endymion and his court resided in a place called The Golden Kingdom, on Earth. Nothing is ever mentioned of Endymion's parents, and little is known about what else went on on Earth during that time, other than events directly linked to the demise of the Moon Kingdom.
The Moon Kingdom was ruled over by a graceful power, Queen Serenity. The heir to her throne was her young and beautiful daughter, Princess Serenity. Blonde, endless hair, white gown, and the sign of the crescent moon on her forehead, she was a beautiful sight to behold. Prince Endymion was very kind, noble, and loving. Although it was forbidden, he fell in love with Princess Serenity, and they often met in secret.
However, times became troublesome when Metallia and Beryl began to turn the people of the Earth against the moon. The people of Earth were swayed and brainwashed by Beryl and Metallia, who got them to hate the moon people for their extremely long lifespans. They cared for the resonate sounds of the Earth - its wind and sea - and yearned for the Moon to be that way. But it never would be, so the Moon Kingdom did its best to keep Earth safe and beautiful as it was, if only to be able to admire it from afar. Beryl made the earthlings forget that the people of the moon admired the Earth from afar, and used their long lifespans to watch over it and guide its evolution.
Prince Endymion knew of the impending invasion on Silver Millenium and fled there to warn his love. Shortly thereafter, the evil attacked, and Endymion fought against his own brainwashed people to protect the moon, and of course, his love.
In the anime version of the battle, Prince Endymion is swept away into a cyclone-type energy that Beryl creates, and Princess Serenity willingly jumps into it after him just to be near him. Beryl then blasts them both, killing them. In the manga, Prince Endymion is killed, and so devastated by this, Princess Serenity turns a sword to her chest and kills herself in grief. In either case, the young lovers died sadly and tragically.
Queen Serenity used the ginzuishou to seal away the Dark Kingdom and to reincarnate her court, her daughter, and the Prince in the future. However, because she was so full of shock and despair, she did not harness the ginzuishou's full power, so the seal she created against Queen Metallia was incomplete. One day, the Dark Kingdom would re-emerge, and her daughter would need to awaken...