Credit to Destined_Chosen on AO3 for coming up with the new title.
"You're not exactly human, are you?" Linhardt asked.
"No, not exactly," Rhea answered honestly. "I suppose I should start with my story. Things will make more sense that way. But before I begin, I want to make one thing absolutely clear. The truth I intend to reveal to you must never leave this room. If the wrong person were to discover this, it could result in countless deaths. All of you must swear to me that you will never reveal this information to another living soul."
"She's not exaggerating," Byleth agreed. "This information has already caused the deaths of thousands, if not millions."
"Whoa, that's a heavy load to drop on someone," Raphael said. "You trust us all that much?"
"Byleth trusts you. That is enough for me," Rhea told them. Each of them gave their promise in turn, they would keep the revelations to come secret.
"Roger, is the room secure? No one is listening in?" Byleth asked.
"No listeners to report, Professor!" the gatekeeper said, entering the room.
"Good," Rhea nodded. "This is the secret history of the Faith.
"Long ago, the Goddess Sothis descended from the heavens to our world. From her blood, she created my people, the Nabateans. Or, as humans called us, the Children of the Goddess. Like the Goddess herself, we all have green hair and eyes, pointed ears, and the ability to transform into dragons."
"So, the dragon at the Battle of Garreg Mach..." Claude ventured.
"Was me, yes."
She expected the others to react with fear, or distrust. Caspar, however, reacted differently. "That's totally badass. Can you turn me into a dragon?"
"...No," Rhea answered.
"Aw," Caspar said, disappointed.
"The Goddess cared deeply for all life, and wished to alleviate human suffering wherever she could. In her day, the most advanced society in the world was Agartha." She told them of how the Goddess had given Agartha a secret that could solve their impending energy crisis, and the Agarthans had attempted to use it to destroy her and conquer the world. How the Nabateans had destroyed Agartha in retaliation, but not before the Agarthans had destroyed much of the world. "The lakes evaporated, the forests burned, lush paradises became barren deserts in mere minutes, cities reduced to craters. The water and even the very air became tainted with ash and poison, survivors of the initial attacks dying slowly of horrible aftereffects."
Nobody spoke as they tried to imagine the devastation Rhea was describing. "Were you there?" Felix asked.
"I was," Rhea confirmed. "Though I was young at the time, and was less involved in the war against Agartha than others. Macuil remembers it better. But regardless, once Agartha was destroyed and every last trace of the weapons that unleashed that hell upon the world had been erased, the Goddess used early all of her power to restore the world. Once she had done so, she fell into a deep slumber. In her place, we resolved to make sure the weapons that devastated the world would never be created again."
"I can see why," Leonie spoke what everyone else was thinking. "What happened next?"
"Agartha was destroyed, but there were survivors. Many of them chose to accept their defeat, but some sought revenge against the Goddess and her Children," Rhea informed, anger slipping into her voice. "They recruited a bandit to do their dirty work. A barbarian named Nemesis, who loved nothing more than to kill." Her hands balled into fists of rage. "He found a way into Zanado, into Sothis's resting place. He tore out her heart and spine, fashioned a sword out of them, and used it to butcher my kin! He used their hearts and bones to create weapons for his own followers!" Her voice broke at the memory.
Many of the listeners recoiled. "So, when I said Friekugel looked like it was made of bones...that's because it was?" Hilda almost screamed.
"Unfortunately, yes," Byleth said.
Hilda looked disgusted. "I'm never touching that thing again."
"No, you should," Rhea contradicted. "My brothers and sisters deserve a chance to fight against their murderers once more. Once we have broken the Agarthans, then they may be laid to rest."
Upon prompting from Byleth, Marianne asked, "The Goddess is dead?"
"Yes and no," Byleth answered. "Her physical form is dead. But she still does have the ability to see and influence the world, just not as directly as she did before. Your prayers didn't go unheard."
"You're proof of that much," Claude said. "What exactly are you, Teach?"
"We'll get to that," Byleth told him.
"I went to war against him," Rhea continued. "Some of you might have guessed as much, but I am Saint Seiros. I slew Nemesis and avenged the Goddess...avenged my mother. I took her Crest Stone back, and attempted many times throughout the years to resurrect her, each attempt ending in failure."
"Wait, if the Crest system was built on such a horrible secret, why leave it in place?" Sylvain asked, a bit angrily.
"I should have. But I was afraid," Rhea told him. "I was convinced that I would be able to bring my mother back, and that once I did so, she would somehow fix everything with minimal bloodshed. A solution that would have led to war, in my mind, would have been blood shed for nothing." She shook her head. "I was a fool.
"But regarding my attempts to bring my mother back to the world, Byleth's mother Sitri was a part of one such attempt. I gave her the Crest of Flames, in the hope that she would be able to bring her back. She was never able to do so, but she was my closest friend all the same. I was ecstatic for her when she fell in love with Jeralt and married him. However, she was always rather frail, and the birth of her child was difficult. Byleth was born with his heart not beating. As she was dying, Sitri begged me to place the Crest of Flames in him to save his life, and I did so.
"I saved Byleth's life, but Jeralt became paranoid over Byleth never crying and having an unbeating heart. And I never told him what I had done because I did not know why Byleth never cried either. I did not know how the Crest of Flames had affected him. Jeralt chose to fake Byleth's death and flee the monestary as a result."
"But I know what effect it had," Byleth said. "I could talk with Sothis directly. I could see her and hear her. She was a close friend. And when Solon tried to seal us away, she granted me her power so I could break us free. As a result, I became a Nabatean as well." He held up his hair, revealing a pointed ear. "And no, Caspar, you can't become a Nabatean. I'm pretty sure what happened to me was an isolated event."
Edelgard nodded. "Solon is as good a place to start my story as any, because he was the one who killed my family. Thales kidnapped me and my siblings during the Insurrection of the Seven, and gave us to Solon to be experimented upon. Solon attempted to implant the Crest of Flames into all of us. I was the only survivor. I don't even know the full extent of what he did to me." Lysithea looked even more horrified than everyone else, Edelgard filed that observation away for later.
"Thales attempted to manipulate me into going to war with the Church, deceiving me into believing the Church and the Nabateans were the root of Fódlan's problems. Sadly, I was so angry at the Crest system and the Goddess that I believed him. I took up the identity of the Flame Emperor to free Fódlan from the influence of the Church and the Agarthans alike. But after the Battle of Garreg Mach, when Rhea discovered she was pregnant and begged me to protect her baby, I realized she wasn't the monster I had thought she was. And that the true tyrant was me. I want to set things right."
"As do I," Rhea said. "The Church has done much harm throughout Fódlan's history because of my poor decisions. For too long I have supported cruelty in the name of peace. It is time to put a stop to it. Will you help us?"
"Absolutely," Lysithea said. "I have a score to settle with these Agarthans."
"So do I," Hapi proclaimed. "I'll teach Thales, Cornelia and all their pals what happens when you mess with us."
"And House Nuvelle shall lend its aid!" Constance added.
"Hell yeah, Ashen Wolves forever," Balthus said.
"To think that House Gloucester could be built on such a dark and bloody secret," Lorenz said. "I swear, I shall do all I can to pay back what my family owes. My lance is yours to command."
"And mine too," Leonie added. "For Jeralt, Remire Village, and everyone else these monsters hurt."
"If you guys are in, then I'm in too!" Raphael shouted. "Let's give those Agarthans a beating they won't forget!"
"I'll help too," Marianne agreed.
"So will I!" Ignatz said.
"I'll help out. These Agarthans don't sound like nice people," Sylvain agreed. "But the Crest system has to go. Deal?" Upon Rhea's agreement, he nodded. "Let's do it."
"Those bastards killed Glenn. I'll make them pay," Felix swore.
"I always wanted to live out an epic tale of chivalry," Ingrid said. "You can count on me."
"And I will do my best!" Annette said as she jumped out of her chair. "I'll show those Agarthans what I've learned!"
"To think that the Agarthans could cause so much suffering," Mercedes said. "They must be stopped."
"And we will stop them!" Ashe proclaimed. "I'll make sure of it!"
"Well, I wouldn't be much of a teacher if I left my students to face danger on their own," Hanneman said. "I will join the fight as well."
"I don't usually like fighting much, but this time I'll make an exception," Hilda smirked. "Let's do this."
"And, if you have any use for a washed-up but still beautiful and talented former opera star, I will be glad to help too," Manuela said cheekily.
"As if I would leave my companions to face the enemy alone!" Alois exclaimed. "I will absolutely help!"
"As will I," Gilbert said. "I will not fail this time."
"And I assume you're getting involved, Catherine?" Shamir asked.
"Of course," Catherine told her.
"Then I guess you need someone to watch your back," Shamir said. "Might as well be me."
"I'll help too," Cyril insisted.
"The Black Eagles will stand with you, Professor," Dorothea told Byleth.
"And I will now report my decision to help out as well!" Roger said as he saluted.
"I'm in too," Anna said. "Normally, my rates are higher than you can afford, but this time it's on the house."
Claude clapped his hands. "Let's make history."