Damnation arrived in the form of a man.
Seiros had left on a diplomatic mission half a day earlier. One of the human settlements on the edge of Nabatean territory had started producing "inflammatory paraphernalia" (it was blatant lies, but calling people out on their bullshit didn't qualify as "diplomacy"). Sothis deemed Seiros as the most capable of her children in matters such as this, so she was sent to try and figure out how to settle this matter peacefully.
She should have known that her mother's sad gaze was the strongest portent of disaster she would ever come to know.
Zanado was already burning by the time she got back. Everywhere she looked, a human was butchering her brethren in cold blood. Those who weren't murdering her family took to pillaging their homes and temples. Some of them were already wielding Nabatean steel against its own forgers. Even those who chose to put on the cloak of the beast were struck down mid-flight by spears of light cast from the minds of man.
Seiros spared no thought for her brothers and sisters. Even as Cichol was comforting Cethleann in his arms, even as Macuil was leading the charge against the bastard mortals who dared to defile this sacred land with sacred blood, even as Indech was getting those who couldn't defend themselves to safety, she had one concern: her mother.
Seiros leapt from the edge of the canyon. Landing with a roll, she sprinted through the stone streets, pushing man and immortal from her path. Her charge persisted through walls of corpses, through slices and stabs that failed to slow her, through the screams of those she once vowed to defend with her life. The shame from this disregarding of her oath would have pushed her to suicide in another time. She had no time for that now. She had to defend her mother.
Her first sight as she entered the temple was a mangled mess of flesh and muscle. The shock stopped her in her tracks. She made a slow advancement towards the corpse, praying against everything that it wasn't her mother, that she wasn't too late.
She could see her mother's dress even through the blood and viscera that stained it.
Seiros' resolve collapsed in her knees first. Her own blood slicked her path as she shuffled towards her mother. Her own cries drowned out those of the massacre outside. Her strength gave out in a domino effect over her mother, breaking her back and neck and arms around the corpse. She could feel the blood soak into her hair and skin.
A low chuckle rumbled through the temple and through Seiros' chest. Seiros slowly and shakily lifted her head from Sothis' corpse to look for the source. Behind her, she saw an old human man leaning against the temple wall. A heavily muscled old man, but the white hair and wrinkled skin gave away his true age well enough. In his hand was a spine.
Seiros' despair gave way to a white-hot rage. She leapt towards the man, hands already extended towards his neck, but he warped away right as she was about to reach him. She crashed her head against the wall, leaving her ears ringing.
It sounded like his laughter.
Redemption came in the form of a boy.
Rhea first saw the homunculus-spawn 20 years after its birth. He had come in with that bastard Jeralt when the house leaders had returned from their ill-fated training expedition. The Black Eagles were in need of a teacher after their teacher turned tail and ran in the battle, and Byleth seemed like a fine-enough teacher, so she let him instruct the house until a suitable replacement could be hired.
The sheer sight of him brought bile to the back of her mouth. This mute, simple, dead-in-the-eyes monstrosity had the gall to survive when his blessed mother had died giving birth to him. She even gave up her own life to make sure he kept his. Rhea couldn't begin to fathom what Sitri saw in the mannequin-like boy.
Fate, however, seemed to love sticking him into her path at every opportunity. The Black Eagles managed to win the Battle of the Eagle and Lion by some stroke of ungodly luck, so they were crowned and lauded as the "guests of honor" at the festival banquet. The house members chose Byleth as the bearer of the medals and trophies as some form of a prank against her. They knew she hated Byleth, and saw fit to make her have to congratulate and crown as a sort of sickening joke. She washed her hands for a long while that night.
And then Byleth just had to go and find evidence of an uprising against the Church. Rhea couldn't risk word of this uprising spread, so she was forced to interact with Byleth to suppress it.
She didn't want to admit it to herself, but she came to tolerate the boy through all of this. She could see that he cared about his students and about her, even through those dead eyes and words whose number she could count on two hands. She even came to look forward to their meetings by the end of it, if for no other reason than to not have someone talk back to her.
Of course, the rebellion came to fruition, and they nearly got to the Sacred Tomb. Byleth was there to stop them, as he has a tendency to do, and somehow got his hands on the Sword of the Creator. Just seeing the sword in mortal hands nearly pushed Rhea into hysterics, never mind the fact that he could wield it. She was inconsolable for days.
After that came a whole slew of incidents that brought Byleth to the forefront of her attention. Miklan Gautier decided to act on his status as "crestless bastard" and steal a Hero's Relic, not knowing the consequences, and so the Black Eagles were sent to deal with it. The Death Knight decided to kidnap Cethleann, not knowing the consequences, and so the Black Eagles were sent to deal with it (Seteth in tow, of course.) Brother Tomas decided to reveal himself as the Agarthan monster Solon and experiment on Remire Village, not knowing the consequences, and so the Black Eagles (with a battalion of the Knights of Seiros) were sent to deal with it.
And then Jeralt died.
Byleth was barely more than catatonic. He got up to eat and drink, and to relieve himself, but that was it. He didn't even move from his bed otherwise. Nearly everyone in the monastery tried to get him to at least look at them, but he only stared blankly at the wall ahead of him. The only person that could get some form of reaction out of him was Rhea herself, and even then it was just him breathing a little faster when she touched him.
He only started acting alive when Rhea told him that they had found his father's killer. He basically threw Rhea out of his way as he sprinted out of the room and towards the Sealed Forest. She made sure to give Alois ample praise afterwards for his swift response in mobilizing the Black Eagles and the Knights of Seiros.
The next time Rhea saw Byleth, he was… different. Past the conspicuous change in hair color, he acted much more human than before. The way he carried himself was much more open, his voice actually had some inflection to it, he seemed interested when his students asked him about something unrelated to classwork. He even nuzzled into Rhea's hand while she sang to his barely-conscious self (why she did that in the first place, she had no idea).
If only Rhea was so composed about the change. Most of her nights were taken up with pleading to the air in the cathedral, begging for Sothis to hear her. She knew that her mother was at least conscious, if not alive, within the boy. She knew that her mother had been the one to save the boy from whatever realm of darkness Solon had decided to send them. She knew that Rhea wanted her to come back so desperately, so why wasn't she just showing herself?! Was she not worthy?! Had she disappointed her somehow?! She was so close to her life's goal, so close to being happy, truly happy, for the first time in a millennia! And the goalpost had just decided to move out of her sight, leaving her without direction.
Rhea had one last, desperate plan. On the last day of Pegasus Moon, she would take Byleth to the Holy Mausoleum and have him sit on Sothis' Throne. Hopefully (based on no actual knowledge whatsoever), Sothis would fully reawaken, and Rhea would finally have her mother back. It could have worked, it should have worked…
...if Edelgard hadn't decided to reveal herself as the Flame Emperor and start stealing Crest Stones from the Holy Mausoleum on that same night.
The resulting battle was a massacre. Half of the Black Eagles were cut down, nearly all of them by a classmate. The conditions of those that survived ranged from "bleeding out" to "catatonic" to "full-on mental breakdown". Rhea's heart hurt for Byleth. She knew all too well what it felt like to have a loved one cut down in front of your very eyes.
Edelgard wasn't able to get a word out to Byleth before he nearly cut her down himself.
The next month would go down in Rhea's memory as one of the hardest in her life. The students (those that survived, that is) were in a constant grief-stricken stupor. The Knights and the other monastery faculty were working around the clock to prepare for the inevitable battle with the Imperial Army. And Byleth was… his old normal.
That hurt worse than everything else combined.
As expected, the Imperial army came, and as expected, the Knights were mostly able to hold them back. What casualties they did sustain were acceptable.
What Rhea wasn't expecting, however, was the arrival of that bastard Thales and his army of mages. His smug smirk turned her calm, battle-hardened focus into rage as pointed as a knife and as indiscriminate as dragon-fire.
Which was only made worse when he pushed Byleth off the cliff.
Salvation welcomed her with the form of a child.
The prisoner had been chained in darkness for as long as she could remember. Her whole world was her body, the manacles on her wrists, and the two pillars that restrained her. Try as she might to break free from the chains, they held strong, much stronger than her.
The only reprieve the prisoner had from the darkness was the stabbing light that came to take her away on some rare occasion. The light bled her and broke her over and over and over again, and yet she always woke up in her chains.
Sometimes she saw shapes from some past life in the darkness. Oftentimes it was a flash of color, dark blue turning into her own pale green, or bone-white to a sickly blood red. Sometimes it was the sound of a sigh, or a feeling of warmth against her hand.
Rarely, it was a word.
Byleth.
This word, "Byleth", aroused a warmth in her chest. It made her feel safe. It made her feel like there was something outside of the darkness and hostile light. It gave her hope.
She was content to just whispering the word some days. When the darkness felt like it was suffocating her, when she felt like she would be stuck in chains forever, she just had to whisper "Byleth", and everything would feel ok again.
When the light came, she would scream and scream and scream, the same word over and over and over. She begged it to banish the light. She begged it to bring back the darkness. She begged it to make everything ok again.
It would take another eternity for the word to save her.
The light came for her once again. She screamed and begged for her word to save her again, but this light didn't try to break her. It broke her chains from her wrists. It caught her when she fell. It held her in its arms. She looked up and saw…
"Byleth."
Then the darkness overcame her again.
When Seiros awoke, she was no longer in chains. She lay on the ground, looking up towards the infinite expanse of darkness that she knew too well. When she moved to get up, a familiar voice called out to her.
"Please, don't move. You still need to heal."
This voice sounded like her. It sounded like a younger her. It sounded like-
"Mother?"
A new shape appeared by her head. A young girl, with billowing green hair and a flowing blue dress. This new shape extended her arm and stroked her hand through Seiros' hair. "Shhhh," she whispered. "I'm here now."
They stayed there for a while, not letting words break the moment. But one word broke through Seiros and rent her to tears.
"Why?"
Sothis' peaceful demeanor turned crestfallen. "Because…" she started. "Because this was the only way." Tears started rolling from her eyes. "I know that I've hurt you, and I know that I've caused you to hurt. But please trust me when I tell you that this was the only way for us both to survive."
"But you-you died!" Seiros cried! "I saw your corpse, I-I saw your bones! You died, and left me with nothing!"
"And if I hadn't died," Sothis said, "they would have come for you, and harvested you the same way they did me." Her voice began to break. "And I couldn't let that happen."
"Then let them! I'm nothing without you!"
"No, no, don't say that, don't ever say that!" Sothis took a moment to gather herself. "If you had died, there would be no chance of me coming back. No-one else was capable of doing what needed to be done!"
Sothis brought Seiros' head to rest upon her lap, running her hand through Seiros' head and wiping away the tears as they came.
"And now, we never have to leave each other ever again."