Chaos was the final villain Eternal Sailor Moon faced. It was said to be the source of all evil in the galaxy and influences others despite not having a physical form for itself. Even when defeated, it is not fully destroyed and is sure to eventually return.
In the manga, Chaos is a being that was trapped in the Galaxy Cauldron and missed out on becoming a star. It sends out incarnations in the form of Sailor Moon's major enemies, and eventually uses Sailor Galaxia to steal Sailor Crystals so it can free itself and rule the galaxy. Chaos does not appear explicitly in the anime, but is a force which possesses Sailor Galaxia.
