Chaos

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, (Queen Metaria, Death Phantom, Pharaoh 90, Queen Nehellenia) and eventually uses Sailor Galaxia to steal Sailor Crystals so it can free itself and rule the galaxy. In apperance she is basically an enormous mass of black fourth-dimensional space. In the end Sailor Moon is able to force it into the Galaxy Cauldron and everyone who was killed are sent back to their originating planet and time, reincarnated after the battle. However it is indicated that Chaos is not dead and will come back as Sailor Chaos much the same as it did in Sailor Cosmos's future.

Sailor Chaos is Chaos reborn. According to Sailor Cosmos, Chaos will emerge from the Galaxy Cauldron reborn as a powerful Sailor Senshi. Sailor Cosmos, frustrated with an ongoing battle against Sailor Chaos, disguised herself as Chibichibi to travel back in time and convince Sailor Moon to destroy the Galaxy Cauldron, and thereby prevent Sailor Chaos emerging to start a new series of devastating wars.

Sailor Chaos never appears in the anime, but Chaos appears very briefly in episode 200 being expelled from Sailor Galaxia after she is defeated. Other than this, Chaos does not appear in the anime series. Sailor Galaxia ultimately reveals herself as the soldier who sealed Chaos within her own body and thus saved the galaxy. Chaos possesses Galaxia in order to give her the power to fight the kibou no hikari - the Light of Hope - Eternal Sailor Moon. But Usagi is able to reach the small bit of hope left in Galaxia's own heart, and she releases Chaos from her body, ending the anime story arc.