They were originally imprisoned into Crystals, which when combined create the Silver Crystal. These were introduced in the Rainbow Crystals Saga, which consisted of 10 episodes.
It is said that if the Rainbow Crystals are gathered by the forces of the Negaverse and the Seven Great Youma awakens, they will revive the full power of the Negaforce and doom the universe.
The Seven Great Youma were the seven very powerful Negamonsters who served under Queen Beryl during her Silver Millennium attempt to overthrow the Moon Kingdom. After the battle, Queen Serenity imprisoned them within seven separate shards of the Ginzuishou which became known as Nijizuishou, or "Rainbow Crystals," because each one was a different color of the rainbow. The Nijizuishou were then sent to Earth, where the Seven Great Youma were reincarnated inside of normal human beings (and in one case, a cat). The humans (and cat) had no knowledge of the Nijizuishou inside of them, although some had special powers such as telekinesis or psychic powers.
Because the youma were sealed within the Nijizuishou, they were unable to exert their evil influence. Once the Nijizuishou were removed, however, the humans reverted to their youma forms. The Seven Great Youma were first introduced in episode 025, in the first season. Queen Beryl instructed Zoisite to locate the seven Nijizuishou in order to form the Ginzuishou and fully revive Queen Metallia with its power.
During the next seven episodes, a massive search ensued. Zoisite was able to locate the Nijizuishou by using the Kurozuisou. He would locate one of the Nijizuishou and extract it from its carrier, causing the person to revert to his Great Youma form. The senshi attempted to stop Zoisite, but were usually attacked by the youma. Rather than killing the youma, Sailor Moon used her Moon Healing Escalation attack to seal the youma within its carrier once again. In episode 034, the Dark Kingdom managed to collect all seven of the Nijizuishou.
However, when Tuxedo Kamen was injured, Sailor Moon began to cry, and the crystals floated to her instead. The seven crystals then joined with her tear in order to form the Maboroshi no Ginzuishou. In episode 041, Queen Beryl sent evil Endymion to capture all seven former Nijizuishou carriers and trap them within the Kurozuishou.
It was hoped that although they had been healed by Sailor Moon, the Dark Kingdom could return them to their youma form and reunite them to form the largest, most powerful Youma who ever existed. Endymion managed to capture five of the former youma before Ryou Urawa caught on when his residual psychic abilities helped him learn that he was the next target. He immediately contacted Ami (who told the other Sailor Senshi) so that she could attempt to stop the Dark Kingdom. Endymion did manage to capture Ryou, but Sailor Mercury snatched the Kurozuishou away from him and threw it to the ground, destroying it and releasing all of the former Nijizuishou carriers. They all returned to their normal lives, never to be bothered by the Dark Kingdom again.
The Seven Great Youma appear only in the anime, never in the manga.
The Seven Shadows
There are the Seven Shadows that have awaken and defeated, and the 7 victims of the crystals have been freed by the Sailor Scouts:
- The Seven Great Youma are an anime exclusive group.