In Manga Volume 12, we begin to learn more about Mamoru's background as Prince. He is not only the Crown Prince of Earth but heir to the Golden Kingdom. In this volume the evil villain Queen Neherenia has cast a curse upon Elysion, all of it has become black and dying. Elysion's priest, Elios has also been poisoned and turned in a pegasus. He contacts Mamoru through a mirror and tells him to find the Golden Crystal and apologizes for not protecting Mamoru. Mamo-chan, too, is slowly being posioned - his lungs have a black shadow.
In Manga volume 13, Mamoru falls unconcious, coughing up black blood, and the history unfolds.
"Mamo-chan!?" says Usagi.
"Don't look," he says.
She sees the blood on his hands. "Mamo-chan!? Black blood!?"
Mamoru kneels on his bed, coughing. The Pegasus stands next to him. All the soldiers show up.
"Black blood!!" says Usagi. "Mamo-chan! How long? Why is your blood black..."
"I don't know," he says. "Since a little while ago. My chest started hurting..."
"Elios!" says Chibi-Usa. "Tell me! You must know! Is Mamo-chan's black blood related to the enemies and you? Is this the enemies' doing!?"
"It seems the time has come already," says the Pegasus. "Let us talk." In a glowing light, the Pegasus transforms into a man, with the same gold horn on his forehead. He kneels before them. "I am Priest Elios of Elysion," he says.
"Priest Elios, of Elysion?" asks Chibi-Usa. "Elysion?"
"Elysion is the holy land that protects Earth," he says. "It quietly rests deep in the Earth, a place like the heart of this planet. I serve as a priest there. My mission is to continue watching over Elysion, and this planet Earth, and you, Prince Endymion. I am also the guardian priest of this planet and you."
"My guardian priest...?" thinks Mamoru.
"Since the distant past," continues Elios, "you have protected this planet all alone. Noble Prince Endymion."
"Do you know me?" he asks.
"Yes, Prince. But you probably don't know me. This planet is protected by the holy land Elysion. I live together with that Elysion. You, this planet, and Elysion and me. Our four spirits and bodies are linked. And those four spirits and bodies are starting to resonate."
"But for that," says Elios, "this is happening." A beam of light shines from his forehead to Mamoru's chest. A dark flower appears in front of him.
"Mamo-chan!?" says Usagi. "Inside your chest...!"
Mamoru looks at himself in the mirror.
"Inside my chest..." he thinks. "A black rose is blooming...!?"
"In my chest as well," says Elios. "A cursed black rose is ruining me. It is a sign of Elysion's ruining. Prince, you are suffering from a curse just like I and Elysion are."
"The form you are looking at is an illusion of me," says Elios. "My body now is also being changed into a Pegasus by the curse. I am confined to Elysion."
Elios tells them how important finding the Golden Crystal is to save Elysion and Mamo-chan. Later when Neherenia is attacking Usagi and Mamoru, Elios saves them by taking them to Elysion.
Usagi looks at the devastated land around her. "This place..."
"This is Elysion."
"Like the wasteland of the night..." thinks Mamoru.
Usagi sees black flowers on the ground. "Mamo-chan!"
"The black roses are a sign of the curse," says Elios. "Of this once beautiful Elysion becoming a land of nightmares. Let us proceed to the shrine."
They enter the old shrine, and climb the steps to the center, where a large crystal formation stands, holding two people inside. Both have two ponytails hanging from balls of hair.
"Is that... crystal?" Usagi says. "And people inside!?"
"Besides me," says Elios, "there are only two inhabitants of this land. Maidens serving the shrine. They are maenads. The crystal is the one hope of purifying this land. These two are sleeping, protected from the curse."
"This is familiar..." thinks Mamoru. "I feel like I know this building, the smell of this place."
"Prince, your kinsmen have always protected this shrine. And Elysion. This is the place where your dead kingdom, the Golden Kingdom, used to be."
"The Golden Kingdom...!" thinks Mamoru. "Yes... I know this place. The same winds as on the surface blow through the thick green of this sacred land that guards this planet, Elysion. I lived in this land, and I loved this place."
"That's right," says Elios. "From here, you went to the surface and protected this planet. Deep in this shrine, I offered my prayers to protect this planet. We never met each other, but our souls were always one. Our wishes were always one. And with the destined meeting of you two, again, we had the same wish. In your separate places, you couldn't be together. Then, too, the two of you shared the same mission, the same power, the same soul."
"The same mission?" says Mamoru. "The same power...the same soul?"
"You protected this planet Earth. The Silver Millennium from without, and the Golden Kingdom from within. And now that mission is carried in the two crystals of your kingdoms. The Mystical Silver Crystal is the holy stone of the Princess of the Silver Millennium. The Golden Crystal is the holy stone of the Prince of the Golden Kingdom. They send out your power."
"The Golden Crystal... is Mamo-chan's crystal!?"
"Here, in the heart of the shrine," says Elios, "is the tower of prayer. This is where I always offered my prayers, and received all sorts of revelations. Those revelations, too, I received at this tower."
Usagi then figures out that inside Mamoru must be the Golden Crystal. Mamoru is shocked - inside him all this time while he felt like he was only dragging Usagi down. But Usagi tells him: "I know it's there," Usagi says. "You've always filled me with strength. That strength always guided the power of my Mystical Silver Crystal. That strength was the very strength of the Golden Crystal! The time has come for that holy stone's power to be crystallized, and released!"
Finally in Manga Volume 15, Mamoru's Golden Crystal emerges! Usagi and Mamoru are hacking up blood and dangerously close to defeat.
"Come on, Mamoru!" Mamoru thinks. "If you don't live, Earth and Elysion won't be saved!"
"I have the power!" says Nephelenia. "You are as good as dust! Be destroyed! Now, everything is mine! I have the unbeatable power! Only I am the true ruler of space!!"
"No!!" Usagi shouts. She grabs Mamoru's outstretched hand. "Mamo-chan!"
"Give me strength, Usa!"
They hold each other and kiss deeply.
"We can't be destroyed here!" Mamoru says.
"Right," says Usagi. "We can't give in to this nightmare!!"
A sphere of light glows around the two. The Mystical Silver Crystal disappears from Nephelenia's hand. Usagi's sailor suit reappears on her body, as she kisses Mamoru.
"My chest is hot..." Usagi says. Light shines from Usagi and Mamoru's chests.
"Power is being born, but... I see."
"No, this can't be!!" shouts Nephelenia.
"Power isn't taken," says Usagi. "It's born. And power isn't born alone. You can't use it alone! I won't lose to anyone! I will show you our power!!"
The golden crescent moon shines on her forehead as she faces Nephelenia.
"Queen Selenity!?" Nephelenia says. "No, damnit!!"
Usagi reaches her arms to the sky, as the light shines above her.
The light spreads all around her. The sailor suits on the other soldiers vanish, replaced by dresses.
Everyone gives their crystal power to Sailor Moon. She transforms into Eternal Sailor Moon, she is also now the Queen Selenity. As everyone gives her their power, Mamoru looks upon her.
"If truly," thinks Mamoru, "the Golden Crystal is inside of me..." He holds his arms to his chest, and another glowing crystal appears. "Give its power to Eternal Sailor Moon!"
As Usagi combines their powers and forms an attack, she kills Neherenia, but not before the latter calls Mamoru the Prince of Earth, protected by the sun.
What happens next is amazing. We witness the coronation of Usagi and Mamoru as King and Queen.
From Manga Volume 15
Elios follows Chibi-Usa, running out of the shrine, and he sees the green land of Elysion, alive once more. The lake sparkles in the sunlight, and flowers bloom all around.
"Prince...!" he says. "Thanks to Sailor Moon, the Golden Crystal was freed."
Mamoru holds out the glowing crystal to Elios.
Elios takes it and fixes it to the top of a regal staff, and hands the staff to Mamoru.
Usagi appears in her flowing white gown, Mamoru in his lavender tuxedo.
Usagi holds out her scepter at the large mirror, and Mamoru holds out his staff.
They draw the evil from the mirror (where Queen Neherenia has been kept), and raise it into the sky, where it is destroyed by the light.
The Princess's four guardian soldiers and the three outer soldiers appear in their royal gowns.
They all stand behind Usagi and Mamoru, who hold their rods.
"Just like a coronation ceremony," says the human Artemis.
"It is, Artemis," says the human Luna.