WARNING: The following contains spoilers for Sailor Moon Eternal: The Movie, now available on Netflix.
Sailor Moon fans are aware of the Silver Crystal and its importance to the Moon Kingdom and Sailor Moon's powers. But what many people don't know is that Tuxedo Mask has his own crystal, one that could rival the power of the Silver Crystal. The Golden Crystal's importance comes into focus in the Sailor Moon Eternal movie, so with the two-parter now on Netflix, let's go over what the Golden Crystal is, where it's hidden and what it's capable of.
The Golden Crystal is the Sailor Crystal for Mamoru. As such, it's the source of all his powers, even when he wasn't aware it existed. The crystal itself relies on the power of dreams to function, so as long as people keep dreaming, the crystal will remain powerful. When everyone on Earth comes together and combines the powers of their sweet dreams, the crystal will be at its maximum power. But the opposite is also true, with the crystal losing power with the more people who don't have sweet dreams.
In the time of the Silver Millennium, Prince Endymion used the power of the Golden Crystal to protect his kingdom, Elysion, home of the crystal and Helios. Helios watched over the Golden Crystal and Prince Endymion as a priest, friend and guardian. When Endymion was killed during the battle at the Moon Kingdom, the Golden Crystal sealed itself away within Mamoru's body. Helios was captured by Queen Nehellenia after she attacked Elysion, and he was able to reach out to Sailor Chibi Moon for help. During the Stars arc, it acts as Mamoru's Star Seed.
Because the Golden Crystal is sealed within Mamoru's body and is so closely tied to the feelings and emotions of the people of Earth, Mamoru's health and powers fluctuate with the crystal's power. In Sailor Moon Eternal, he becomes sick when people's dreams are being corrupted by Nehellenia's minions; even the health of the planet itself is dependent on the crystal's power. The seal is only removed when the Silver Crystal evolves into the Silver Moon Crystal. It, along with the Sailor Crystals of the other Senshi, allow Sailor Moon to transform into her Eternal form by the film's end.
It's at this time that the full potential of the Golden Crystal is reached. Helios then takes it to Elysion and fixes it to the top of Endymion's staff, which transforms Mamoru and Usagi into King Endymion and Neo-Queen Serenity.
The Golden Crystal is able to power up others and allow beings to maintain human-like forms, such as in the case of the Amazon Trio. The crystal is also how Sailor Moon receives the Moon Kaleidoscope and Sailor Chibi Moon receives the Crystal Carillon. The power of the Golden Crystal rivals and complements that of its silver counterpart, increasing its power to an exponential extent. When released, it takes on a lotus-shaped appearance, much like Sailor Moon's crystal. It also appears to have its own sentience, with thoughts and feelings of its own, while allowing a being with sweet dreams and pure intentions to wield its power and shields them from even the most powerful attacks. And who better to do that than Mamoru?