I am Chaos. I am the one who never got to become a star. The one who instead became the ruler of the cauldron, the dark birth place of the stars.
So that's it! You intend to seal me away by force using that sword, just as she once did! But power alone cannot defeat me... for this body belongs to Sailor Galaxia, the most powerful Guardian in the galaxy!
Now that you've chosen not to fight, and forfeited your pride as a Guardian, what can you possibly do? All that's left for you to do is to hand your light over to me, and then fade quietly out of existence!!
~ Chaos during the final battle against Sailor Moon.
Chaos is the ultimate antagonist and final enemy in the Sailor Moon series. It has little to no true form. Regardless, its agenda is destruction.
In the manga, Chaos resided in the Galaxy Cauldron, the center of the galaxy where everything was formed from it. Its desire was to bring everything back to it and become one with everything in the universe. It had manipulated, instead of controlled, Sailor Galaxia into stealing Sailor Crystals. When Sailor Moon encounterd 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 and she survived only with the help of Sailor Moon. It 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 it, Chaos would return in the future, but it also meant new life would be born, ensuring the galaxy's future.
In the anime, Sailor Galaxia battled against Chaos. In order to defeat it, Galaxia sealed it within herself to save the galaxy. Eventually, Chaos gained control over her and started stealing Star Seeds throughout the galaxy. In the last episode, Sailor Moon frees Galaxia from its control and returned Chaos to its rightful place within the hearts of everyone where it could no longer do any harm.
Chaos is named after Chaos from Greek mythology. Chaos was a primordial god, a dark shapeless void, and the original state of existence from which the first entities appeared. The mythological Chaos was neither good nor evil, and was rather just the ultimate expression of nothingness and the antithesis of Cosmos.
Chaos' concept as a force of evil whose repeated incarnations serve as the main foes of the Sailor Scouts is similar to the Great Leader from the Kamen Rider franchise, whose incarnations repeatedly menaced the Kamen Riders.
In the manga, Chaos can be considered as the overarching antagonist of the series due to the fact all the major villains are incarnations of Chaos. However, this fact is not established in the 90's anime.