Seven Great Youma

The Seven Great Youma, also known as the Seven Shadows, are a group of 7 powerful Negamonsters with the energy to reawaken the Negaforce.

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 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:
 * Gesen
 * Boxy
 * Bunbo
 * Binah
 * Rikoukeidar
 * Jiji
 * Bakene

Trivia

 * The Seven Great Youma are an anime exclusive group.