"I am Chaos. The one who missed becoming a star. And the ruler of this Cauldron's stars of darkness." - Chaos, Act 51
Chaos, not Galaxia, is the main enemy of the Stars arc, even though we don't meet him until Act 51. Galaxia is just his pawn; she was fooled into helping him. His ultimate goal is to absorb the energy of all the galaxy's senshi into the Galaxy Cauldron, which he can then control - without anyone to oppose him.
Like all of the other major enemies of the Sailormoon series, Chaos is formless - a cloud of inky black, with three bright points glittering like stars from its mass. He (I'm calling him "he" just for lack of information. "He" could be a "she" for all I know, and "it," while accurate, is quite awkward.) seems able to send out blasts of debilitating energy.
Apparently, all of Usagi's greatest enemies - Metallia, Death Phantom, Pharoah 90, Nephelenia... they had all been shadow-selves of Chaos, his alter egos. All his siblings having failed, he turned his attention now to Sailormoon - and her ultimate power. As he pointed out, light called to the darkness... and darkness called to the light... he and Sailormoon, ultimate darkness and ultimate light, were well-matched. He invited Sailormoon to attack him, giving him her power.
Usagi did one better. She joined him. She dove into the Cauldron, summoning the energy of all her friends across the galaxy - and shot it at Chaos. He couldn't handle the barrage of sailor crystals, and screamed... and was melted back into the Cauldron, into the sea of endless possibility.
It's implied that Chaos was reborn, in the far future, as Sailor Chaos, who fought against Sailor Cosmos... but that's another story.
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