Dream mirror

A dream mirror is a mirror within a person that represents the physical form of their ability to dream.

Sailor Moon Super S
The Amazon Trio were sent to search through their targets' dream mirrors for Pegasus, as Zirconia knew he was hiding in someone's dreams, but they did not know who. They would target a different person (based on pictures taken by Zirconia's pet, Zircon) and extract their dream mirror. They would then look into the dream mirror to find Pegasus. Doing this appears to cause the victim agonizing pain.

Fish Eye soon began to question Zirconia's ways, as she was aware that she, Tiger's Eye, and Hawk's Eye did not have dream mirrors (at one point, she tries to perform an extraction on Tiger's Eye, but it doesn't work as there is no mirror to extract).

Serena's dream mirror was extracted by Hawk's Eye and destroyed by Mr. Magic Pierrot, knocking Serena out and nearly killing her, until Tiger's Eye and Fish Eye gave up their power to rebuild the dream mirror so Serena could transform into Sailor Moon and destroy Mr. Magic Pierrot. All three of the trio were killed, but Pegasus revived them and gave them their own dream mirrors.

Afterward, Queen Nehelenia told Zirconia that they would know the holder of Pegasus's dream mirror when they found it because it would be a golden mirror. Whenever the Amazon Quartet used their orbs (like balls in pool) to extract dream mirrors, they would know that their victim's dream mirror was not the golden mirror. They would usually send a Remless to eat it, but it would always be returned unharmed. One of the victims, an artist, had his dream mirror corrupted because Selesele forced him to not follow his heart. Togetoge, the Remless, refused to eat it because of this.

Rini's dream mirror was eventually extracted by Zirconia himself, using Zircon as an orb, and was revealed to them to be the golden mirror.

Trivia

 * Using a mirror to view a person's dreams is also a trope used in Dragonball Z.