Chaos

Chaos is a major villain and final enemy in the Sailor Moon series.

Anime
In the anime, Sailor Galaxia battled against Chaos. In order to defeat him, Galaxia sealed him within herself to save the entire galaxy. Eventually, Chaos gained control over her and started stealing Star Seeds throughout the galaxy. In the last and final episode, Sailor Moon frees Galaxia from his control and returned Chaos to his rightful place within the hearts of everyone where he could no longer do any harm.

Manga
In the manga, Chaos resided in the Galaxy Cauldron, the center of the galaxy where everything was formed from. His desire was to bring everything back to him and become one with everything in the universe. He had manipulated, instead of controlled, Sailor Galaxia into stealing Sailor Crystals. When Sailor Moon encountered Chaos, Sailor Galaxia believed the confrontation between Chaos and Sailor Moon would kill them both, allowing her to be the ultimate Sailor Senshi, but Chaos immediately attacked Galaxia in anger and she survived only with the help of Sailor Moon. He was revealed to Sailor Moon that in order to permanently destroy Chaos, she would have to destroy the Galaxy Cauldron, the source of all life, including Chaos. Rather than destroying both Chaos and the Cauldron, Sailor Moon jumped into the Cauldron, and using the Silver Crystal, she dragged Chaos down with her, destroying him. Since Sailor Moon chose to preserve the Galaxy Cauldron, rather than destroy him, Chaos would return in the future, but it also meant new life would be born, ensuring the galaxy's future.

All of the previous main villains in the manga (Queen Metalia, Wiseman, Pharaoh 90, and Queen Nehelenia) were all incarnations of Chaos.