The Purity Chalice is a cup with extraordinary power. It can only appear if three specific Pure Heart Crystals (a saber called the Space Sword, a mirror called the Deep Aqua Mirror, and a crystal ball known as the Garnet Orb) are brought together.
For the first 22 episodes of Sailor Moon S, the Heart Snatchers are looking for the Purity Chalice so they can use it to summon Pharaoh 90 from another galaxy. Once the three Pure Hearts were brought together (those of Sailor Neptune and Sailor Uranus, and Sailor Pluto's crystal ball from the Garnet Rod was the third), the Purity Chalice appeared. Sailor Moon and Eugeal raced for it, and Sailor Moon got it. She used it to transform into Super Sailor Moon and defeat Eugeal.
From here, Sailor Moon would always use it to transform into Super Sailor Moon and then use Rainbow Moon Heart Ache to destroy Daimons. This was also used against Telulu to destroy her plants, and against Byruit to damage her nanocuff, resulting in her death.
Mistress 9 tricked Sailor Moon into giving the Purity Chalice to her, and she used it to summon Pharaoh 90. The Purity Chalice was destroyed in the process, then Mistress 9 was betrayed by Pharaoh 90 as he had no further use for her after she completed her part in delivering the Purity Chalice to him.