The Black Moon is the menace from the future. Quick rundown: Sailormoon and Tuxedo Mask (aka Usagi and Mamoru) become the rulers of Earth. Neo Queen Serenity and Neo King Endymion rule long and well, and even have a young daughter, Small Lady Serenity (aka Chibiusa). By the time the 30th century rolls around, they've built a place of beauty and peace on Earth, named Crystal Tokyo. Suddenly, they were attacked - a huge explosion and a flood of poison gas caught the rulers off guard. The senshi and Neo King Endymion were defeated, and Neo Queen Serenity was enclosed in crystal... but their daughter Chibiusa remained unaffected. Chibiusa stole a space-time key from her friend Sailorpluto, and traveled to the past with the intention of bringing the legendary Sailormoon to the future.
Nemesis, the mystical planet of darkness, is the tenth planet of the solar system. For centuries, criminals were sent there in place of capital punishment. On that planet, the descendants of traitors came to power - and called themselves the Black Moon, a conscious effort to be opposite to the bright power of the Earth. They created the Evil Black Crystal, a huge monument of negative power. Influenced by the cloaked shadow-creature that called itself Wiseman, four young traitors traveled to Nemesis, became powerful, and attacked Crystal Tokyo. Having succeeded, they used the power of the Evil Black Crystal to send their agents backwards in time. Then, they could neutralize the Sailor senshi while they were younger, and not as well trained - and capture the young time traveler Chibiusa, who had originally escaped their grip.
The young traitors are Prince Diamaund, his brother Safire, another young man named Rubeus, and a woman named Esmeraude. They utilized the talents of the Ayakashi sisters and various lesser evils to aid them in defeating the senshi.
The power behind the rise of the Black Moon was the creature who called himself Wiseman. He was the one who fooled Chibiusa into trusting him - and then he made her into the strongest of the Black Moon pawns, the Black Lady.
Hasidia points out an interesting Black Moon fact about the four main Black Moon villains. She says that "their names are, in a way, connected to the four senshi captured: Moon, Jupiter, Mercury, and Mars. They are Diamondo (Diamond) Usagi's favourite stone (also said to be Chibiusa's, another senshi also captured!), Esmeraude (Emerald) - Makoto's favourite stone (and the one in her tiara), Saffiru (Saphire) - Ami's favourite stone (and the one in her tiara), and Rubeus (Ruby) - Rei's favourite stone (and the one in her tiara). I noticed that the only inner senshi not captured is Venus - and I also noticed that there is no villian with a name that is a variation of the word 'Topaz' (Minako's favourite stone, and the one in her tiara)."