...on the other side of a mirror, another, pitch-black world exists. So on pitch-black nights, of new moons, you can't look into a mirror. Otherwise, you'll be drawn into the world inside it.
The Dead Moon Circus folk are the cutest villains of the entire series. It seems as if Takeuchi-sensei decided to give us a break from the grim rapacity of the Infinity arc and romp through a gentle fairy tale. The Dream arc of the manga features self-discovery and the power of love.
When the light of the sun was eclipsed by the Moon, the creatures of the Dead Moon saw an opening to gain access to the Earth. Pretending to be an exotic circus, they quickly established a presence in Tokyo. Their circus performances drew large audiences, from which they were able to drain energy. Their goals were to steal the senshi's dreams and spread nightmares across the people of the Earth.
To understand the motivation behind the Dead Moon Circus' attack, you need to know a significant bit of history. The major power behind the Dead Moon is an ancient, bitter woman: Nephelenia, the self-styled Queen of the Moon's dark center. She believed that she was the one truly meant to wield the ginzuishou. She sent her minion Zirconia to Earth to find it. Zirconia brought the four mischievous girls of the Amazoness Quartet, who helpfully created the Amazon Trio. Together, they wreaked havoc while the senshi desperately tried to figure out what was going on. Lesser evils also helped out, and various lemures came along for the ride.
During the Circus' attack, specific balances were being disrupted, and Usagi and Mamoru were put at a significant disadvantage. The only way that they were able to overcome the attack from within is through the power of their love and the united potency of their respective Silver and Golden Crystals.