The denizens of Garreg Mach are overwhelmed when Edelgard tipped her hand and revealed hundreds more soldiers prepared to march on the Monastery. Fighting had become a lost cause. Everyone left on the battlefield retreated and prepared to board the few remaining caravans fleeing to the Leicester Alliance and the Kingdom of Faerghus.
Byleth remains on the hilltop adjacent to the Monastery, however, and watches as armed soldiers swarm out of the town where they'd made their last stand and ascend like waves of ants to a toppled picnic.
The Professor palms the hilt of his etched-bone blade, prepared to knowingly risk everything if necessary.
But then a gentle hand on his shoulder stops him from advancing.
Archbishop Rhea, her brighter green hair even more iridescent in the golden hues of the setting sun, passes in front of the stone-faced instructor. Her gait is steady, calm in the face of a storm that would undoubtedly dismantle everything she held dear. Even her thin-lipped smile is unusually perky as she glances back toward Byleth over her shoulder.
Her words are as collected: "Everything here, young and old, is in your hands."
Byleth's grip on his sword tightens. Some part of him had never fully come to trust Rhea's grandiose presentation and her iron-grip on any and all information that passed through Garreg Mach. Yet he could see in her eyes that same willingness to risk everything to protect the remaining students and faculty.
Her comment also struck true. If anyone knew anything about what Byleth had gone through the last few months, it would be her. She knew of the power he had inherited, and how careless it would be to throw it away at the inception of the war.
So, with a reluctant breath out through his nose, he lets go of the Sword of the Creator and nods.
As the Archbishop turns to face the precipice of the cliff overlooking Edelgard's approaching army, Byleth could feel a tug at his gut. A voice in his head sounds off, telling him to run. Something was about to happen, and he didn't want to be there.
He turns and runs, black jacket rippling behind him as Rhea closes her eyes.
"No. I will not allow another Red Canyon tragedy to happen here," she mutters.
Suddenly her entire body is enveloped in a grassy emerald light that expands out until the woman was replaced by a star locked to the Earth.
When the light contracts again, Rhea is gone. In her place is an immense dragon, white as eggshells, with a wingspan the size of eight city blocks and a line of charcoal spikes running up its spine that match the four horns jutting from its forehead and snout.
The beast roars and the ground shakes around it, the very act splintering the remaining strands of skin toward the edges of its maw so rows of jagged teeth could be exposed. A shimmering green aura continues to encompass its figure as the dragon flaps its wings and takes off, but the aura fades by the time it was halfway to the town.
Byleth makes his way to the remaining front lines holding off advanced scouting troops to usher students toward their fleeing caravans. The house leaders, Dimitri and Claude, are some of the few youths who kept their weapons.
However, only Claude stops in awe of Rhea touching down in the middle of the advancing army. Her scaly talons immediately flatten and grind a number of soldiers into the dirt, and more scatter as she raises her elongated draconic neck to the sky for a spine-tingling roar.
"What is going on?" He asks with a hand up to the side of his head, trying to comprehend exactly what the Archbishop had become.
Dimitri was lost to his slaughter, still dinged up from his encounter with Edelgard but nowhere near ready to give in.
Byleth only turns to watch Rhea's bestial form when he can sense the energy she begins charging like an orb of fire between her jagged teeth. Again a voice spoke deep inside him, and he knew he was witnessing the true power of an ancient being he had only read about in the library: The Immaculate One.
The Immaculate One lets loose the orb of energy as a continuous beam that fires off when she slams her tapered tail against the ground like a drum. As it passes through the army and the town that made their backdrop, everything is vaporized in holy fire.
As buildings crumble, a pack of grounded Demonic Beasts rush forth through the rising cloud of dust. These beasts were of the same experimental ilk as the Adrestian Empire had been utilizing, but adorned with the same stone mask as the flying foes that Hubert controlled in the first wave. They were trained, and clearly trained to kill Rhea from how ceaselessly they bolt toward her.
Multiple Demonic Beasts manage to evade her swipe and leap onto the dragon three times their size, eventually overwhelming her and sending The Immaculate One tumbling into the cobbled wall that separated this town from a bottomless chasm.
Byleth knew he should remain to help the students, but feels an almost maternal instinct drive him toward the battle of the colossi. Once he's close enough, it only takes one thrust of his extending, chain-like weapon to shatter the mask of one of the monstrosities in a scarlet burst, as well as destroy the Crest Stone beneath it.
Once that monster falls off The Immaculate One, she is able to shake off the rest with the help of debris coming down off the collapsing wall. A few Demonic Beasts fall into the chasm and disappear into its pitch-black depths.
She turns to Byleth and stands on her hind legs, long neck craned down so their gazes could meet. Despite this new form, Byleth could feel Rhea's presence in her elliptical pupils. They were undoubtedly one-in-the-same.
But instead of giving thanks, The Immaculate One's booming voice asks, "Why did you come?"
In that moment, Byleth's otherwise blank expression comes to life with a gasp of surprise. He knew what she was afraid of, and it seems they were both too late to stop it.
The Professor quickly looks back over his shoulder to find a band of cloaked figures suddenly standing out at the peak of a nearby hill. At the forefront was Thales, whose pale white eyes surrounded by ashy black paintbrush lines and occult earrings bearing the circular shape of the group's insignia were immediately recognizable.
It was the same man who prevented Byleth from saving his father from Kronya those months ago. There was no way he could forget that face.
Before he can make any moves against the dark figure, Byleth is struck by Thales' singular blast of thick purple energy. Rather than exploding, the orb instead catches on Byleth's armor and sends him flying through the air.
The Immaculate One moves to catch him, but is stopped by another wave of Demonic Beasts. All she can do is watch in whatever constitutes as terror on the face of a draconic entity as the blast sends Byleth careening into the mouth of the chasm.
It takes Byleth a moment to regain his faculties before he realizes that the ground underneath him was gone. The man plummets, at first silent as his voice catches in his throat. Then he screams, approaching terminal velocity fast as the world of light above him begins to blend into a fuzzy haze with the dark rocks of the surrounding cliffs.
It doesn't take long for his screams to be drowned out by the roaring winds that cut past his armor as he fell, and soon the sound mixed with the strain of gravity on his figure to become a completely overwhelming sensation. The Professor closes his eyes, waiting to feel nothing.
And then, everything goes dead quiet.
Yet… Byleth wasn't dead yet.
It was as if he was floating in an incubation chamber, and suddenly he could feel hands press against the chest plate of his armor.
"Sleep, Byleth."
The voice echoes in his ear, so quiet and ethereal that he wasn't sure if it was real.
When he opens his eyes he finds the light far above him completely blocked off by a silhouette of long, green hair. He couldn't see her face, but now he could place Sothis' voice.
"You have all of my powers, so you knew this was destined to be." Sothis sounds completely calm, her presence stifling the chaos of his descent. "Your time is yet to come, you will rejoin this fight."
"But…" Byleth barely manages to start before a lithe finger against his lips silences him.
"But nothing," Sothis says. "Sleep. You will be the end of all this in due time."
With that Byleth could feel his body shut down on him. His eyes flutter closed once more, and he's enveloped by the bleak shade of the canyon's depths.
As Garreg Mach burns in the distance, a rickety collective of wagons roll lazily down the long road to the Leicester Alliance.
In the back of one sits a girl with pink ponytails, legs curled to her chest as she watches the driver, and another with short blue hair who stares out onto the rising red plumage and the dark clouds it casts.
"I can't believe it had to end like this," Hilda manages with a hoarse voice before they hit a rough bump in the road, causing her relic weapon to jostle against the floor.
Marianne is silent, and Hilda only waits a moment for a possible response before continuing.
"Edelgard… Who would've guessed she was such a menace. Can't believe I gave that girl a hairpin." She brings a slight smile to her face before glancing over at Marianne. "I hope she pricks herself with it now, if she hasn't just thrown it away."
The other girl continues to say nothing as she burns the image of the flames into her retinas.
Hilda sighs and reaches out, resting a hand on her friend's side.
"You're going to kill your neck doing that, Marianne."
When the blue-haired girl finally looks to Hilda, she gestures for her come over.
Marianne relents and flops back onto her rear so the girls can snuggle in. Hilda starts to look over some of the lightly bleeding scars that sullied Marianne's outfit as the quiet girl closes her eyes.
"Did you see Dimitri out there?" Hilda ventures, not quite as oblivious to Marianne's circle of close companions as she hoped the fellow noble would be. "I never would have imagined him capable of such strength. Or anger, for that matter."
The cleric sniffles back whatever her throbbing heart wanted to say and shakes her head.
"I've known him to be capable of it from the start," she remarks sadly.
Hilda looks surprised, but Marianne keeps on.
"But I know he's capable of so much better too."
In Imperial Year 1181, the new Adrestian emperor, Edelgard von Hresvelg, led a strategic assault against the monastery at Garreg Moch. Though her own losses were great, her foes had no choice but to surrender.
Archbishop Rhea commanded the Knights of Seiros, leading from the front lines against the Imperial army. But amidst the chaos of battle, she vanished, and her whereabouts remain unknown.
With this single attack, the Adrestian Empire officially launched its offensive against the Holy Kingdom of Faerghus and the Leicester Alliance. The unification of Fódlan has begun.