A/N: I'm sure with the title, all of you can guess where I'm at with Fire Emblem Three Houses currently. It's an idea that just hit me recently and I wanted to get this out ASAP. Best of all, it's quick and none of you have to wait several months for a chapter of another certain story.

Disclaimer: I do not own Fire Emblem Three Houses

Vow of the Two Flames

Darkness. That was all Byleth could ever see. Or rather, feel. It felt like she was floating throughout this darkness while it passed her by. Unlike the darkness spell Solon has cast to send her and Sothis away, this darkness felt more...peaceful. However, every now and then, she would see images of a battle and hear them too. Images of her and others fighting against a massive, powerful army. What stood out to her the most was a large white dragon that'd appeared out of nowhere and fought alongside them. She had fought against a young girl with long pale hair who donned flaming red armor and felt a myriad of emotions coursing through her body. The dream ends with Byleth helping the dragon before being struck by a strong magic attack cast by a being with ghostly pale skin. The attack pushed her away from the dragon and tried to head back to it, but the ground crumbled beneath her and fell screaming while falling into darkness. That is where the dream ends and she would see it again and again and again.

What was the purpose of this dream? Why would Byleth see these events countless times over? Most importantly, why did she rush over to aid the dragon? She had asked these questions time and again, but the answers always evaded her. She continually drifted through the darkness and did nothing more. That is until she saw a bright green light with a mysterious mist surrounding it. From the mist, she heard of a familiar voice of a young girl. "You...how long do you intend to sleep?"

Sleep? Was Byleth sleeping? "Your body is awake. Your eyes must open now, and you must find the strength to stand upon those legs of yours. Like so much rain, a pool of blood has fallen to the ground as spears and arrows pierce the earth, it weeps. And even now...it weeps. In order to survive, they kill. And so, the people of this world are lost in an abyss of suffering. They weep as well. The only one who truly knows the nature of such thing is I. Or rather, you."

All Byleth could do as a response was groan whimperingly "I'm still sleepy."

Byleth heard the voice sigh frustratedly which also felt and sounded familiar to her. "You are a complete and utter fool! Have you not changed one bit?! Get on your feet, right now! I'll coddle you no more! You are just like a child, always needing me to hold your hand." The green light grew bright and it enveloped Byleth entirely.

D

"Hey, a-are you awake?"

Byleth groaned as she finally opened her eyes and found herself lying on the cold ground. She stood up and the first thing she saw was a villager and judging from her surroundings, she found herself in a ruined village. She groaned again while holding the side of her head with her hand as a means to quell a slight pain that was currently bothering her now. "Where am I?"

"We're in a village at the base of the monastery. What are you doing in a place like this? I honestly didn't expect to find someone floating down the river." Floating in a river? That explained why it felt like Byleth was floating and why her clothes were dripping wet. "Garreg Mach is upstream, but that place was abandoned." Garreg Mach. For a second, that name sounded familiar to her. Then she remembered. Garreg Mach Monastery was the place where she was hired to become a teacher and had taught a group of students. More specifically, the Black Eagle House.

Hearing the monastery abandoned was news to her. "What do you mean?"

The villager seemed confused by Byleth's question. "Huh? You don't know? The Church of Seiros isn't there anymore. Though, there have been some folks still living there in the five years since...well, you know. Anyway, I've heard some thieves have been spotted around those parts these days."

Byleth quickly caught what the villager had said earlier about how it's been five years. She became further confused now as she removed her hand from her head. "Five years?"

The villager became just as confused as Byleth. "Um, are you feeling alright? You didn't hit your head or anything, did you? It's the Ethereal Moon of the year 1185. It's been nearly five years since the monastery fell. Tomorrow was supposed to be the Millennium Festival, but who's got time to think about things like that?"

"The Millennium Fesitval..."

Hearing about that stirred something inside Byleth. It brought back an old memory. A memory where a certain pale-haired former princess promised all of them would meet again five years later of when the Millennium Festival would happen. "I have a proposition. Let's all agree to meet back at the monastery exactly five years from today. Who knows where each of us will be in five years or who we will become? Still, I have faith that all of us will gather and celebrate our reunion." Looking back on it, it was ironic to Byleth that she would make such a promise and then later discovered she was the one responsible for everything that has happened to her and her class.

"Uh, yeah. That's what I said. But with the war and the archbishop still missing and all...I doubt there's a soul to be found who has enough blessings worth counting." Byleth had heard all she needed to. During this short conversation, she remembered everything. About the promise, about the pale-haired girl who was with her, about the battle she took part in defending the monastery with her students, about the white dragon that had appeared, and how found herself sleeping for five years. There was only one place for her to go now. She turned away from the villager and was about to walk away and he called out to her. "Hey, slow down, will ya? Where do you think you're going?"

"The monastery," Byleth replied in a calm, cool voice.

The villager thought she was joking at first, at least he hoped she was joking, but quickly found out she was serious. "Are you crazy!? I told ya, they say thieves are running amok up there, there's plenty of other dangers too. Come on, I promise I won't say you're a coward. Just forget about going anywhere near the monastery." He saw Byleth shake her head and gave up. He knew there was no chance of changing her mind. "You just remember I tried to stop you, got it? It's not on my conscience if you wind up dead."

Byleth closed her eyes and smirked a little. It was obvious the villager was worried about her, and she was grateful for that. "No need to worry about me. Thank you...for pulling me out of the river." She started to walk away from the villager and headed toward the monastery. Where her students were waiting for her.

As the villager saw Byleth walking away from him, all he could do was sigh and shake his head. "Unbelievable"

B

It was as the villager said, Garrag Mach was in ruins. It was as if Byleth was looking at a shadow of the monastery's former self. As she approached it, memories began to flood into her. Of how she and her father, Jeralt first arrived, meeting everyone from the Black Eagles, the Golden Deer, the Blue Lions, and learning about all of them. Flayn's abduction, fighting the Death Knight, obtaining the Sword of the Creator for the first time, the Battle of the Eagle and Lion, the White Heron Cup, the ball they had, and how Byleth's appearance changed when she and Sothis merged and escaped the darkness Solon had banished them. Out of all the memories, there was one that stuck to her the most. Of when she and Edelgard ran into each other at the Goddess Tower right after Byleth left to get away from the festivities of the ball.

It was there Edelgard shared her story of how her parents first met and how they fell in love. Hearing that reminded Byleth of a rumor that if a man and woman made their vows to each other on the eve of the ball, they would be blessed by the Goddess and be together forever. Byleth and Edelgard never did make a vow to each other, but she had a feeling it wasn't a coincidence that they met there on that very night. "The Goddess Tower..."

For some reason, Byleth headed over the Goddess Tower as if she was being pulled toward it. As she approached it, she saw that it was undamaged despite what had occurred five years earlier. She slowly made her up the steps with each step echoing throughout the staircase. And with each step, her mind focused more on Edelgard. She remembered something else Edelgard has said to her on the eve of the ball. "Don't forget, my teacher, even if the Millennium Festival should be canceled, I promise to return here"

"Are you really here, Edelgard?" Byleth would receive her answer soon enough, but a part of her hoped it was true.

Byleth had reached the top of the stairs and was instantly greeted by the sight of a lone woman wearing a red cape with a twin-headed eagle on the back and cowled dress. What caught her attention the most was her pale hair in the style of hair buns adorned by a curved hornlike crown. She heard the woman sigh to herself. "Why am I even here? I know they won't come, but here I am anyway. How has it already been five years? Time stops for one, I suppose."

"Edelgard..."

Edelgard's eyes widened with surprise and turned around to see Byleth, unchanged since the last time they fought despite five years had passed. "Huh? It's..it's you!" As she turned around, Byleth saw how much her former student had changed in five years. She was not the young girl she taught in class five years ago, but now a beautiful woman who was the Emperor of the Adrestian Empire. "What are you doing here?"

Only one answer came to Byleth's mind. "Just on a whim."

Edelgard was taken aback a bit with Byleth's answer but quickly regained her composure. "A whim? Highly doubtful." It was still strange seeing her former teacher again after all this time. "So, you were alive. What have you been doing all this time?"

Byleth had no choice but, to tell the truth. She could hardly believe it herself it happened to her. "I was sleeping."

Edelgard couldn't help but facepalm at the answer. "Joking at a time like this..." It appeared that her professor had not changed at all since she occasionally joked every now and then. At times they were humorous, others with not good timing." She removed her hand from her face and looked directly at the former mercenary. "Well, if you don't wish to tell me, I won't try to persuade you. All that matters is this; will you return to the Empire with me?"

Byleth closed her eyes. "I can't." She opened her eyes and there was a hint of anger in them. "You were responsible for Flayn's kidnapping, for what happened at Remire Village, for killing my father, and for attacking the monastery. I cannot and will not join someone the likes of you."

Edelgard felt pain in her heart when she heard Byleth's answer and the way she was looking at her. She slightly lowered her head. "As expected. I knew your answer before I had even asked the question." She knew the answer, but that didn't lessen the pain within her heart. She raised her head and her eyes were filled with determination. "That means...we're enemies, you and I. My teacher, the time for discussion has come to an end."

The two women stared at each other and Edelgard drew out her sword. Byleth gasped quietly as she recognized the sword Edelgard was wielding. "That's...Rhea's sword. Did Edelgard already...?" She had to dismiss the thought quickly as Edelgard ran toward her and Byleth drew out the Sword of the Creator. Edelgard swung her sword at Byleth and her former teacher did the same.

The two swords struck each other neither of them was moving an inch. "Even though our swords may cross as they do now..." Edelgard moved sword upward to make an opening and swung her sword again at Byleth. The former mercenary managed to block the incoming attack and the two women exchanged sword clash after sword clash at each other. After their swords struck each other once more, they both pulled back and turned around with each of their swords a hair's distance away from their necks. They stared each other down, unflinching until Edelagard's eyes softened. "There's no denying that our chosen paths never will." She moved away from Byleth as her former professor withdrew her sword. Edelgard turned away and sheathed her sword as did Byleth. "Goodbye, my teacher. When next we meet, one of us will breathe their last." She began to walk away from Byleth.

"Edelgard..." The emperor stopped as soon as Byleth called out her name. "Before you go, I want to give you something." She walked toward the other woman and Edelgard turned around to face her former teacher once more. Byleth stopped in front of Edelgard and presented something to her in her hand. "I believe this belongs to you."

Edelgard looked down and saw a small red pendant with the Black Eagle insignia engraved on it. "You kept it." She took the pendant and held it carefully in the palm of her hand. As she looked at the pendant, Edelgard felt her heart twinge with pain once more. It shouldn't surprise her that her former professor would return the gift she gave to her on her birthday. After all, who would keep a present from the person who was involved with terrible deeds done?

As Edelgard stared at the pendant, Byleth thought now was a good time as any as this would be the last chance she had and the last time they'll see each other before fighting one another. "There's something else I want to give you as well." Byleth brought her hand under Edelgard's chin and lifted it slightly so they could look at each other once more. Byleth closed her eyes and before Edelgard could say something, her mouth was suddenly silenced by something soft pressing against her lips. Her eyes widened in shock as it took a few seconds to register that her professor was giving her a soft, gentle kiss. It took a few seconds for the shock to wear off and Edelgard slowly closed her eyes and tried to enjoy the kiss for as long as she could. A few moments felt like an eternity to them as they stood still while the moonlight basked upon them. Byleth guessed at this time, her heart would be beating madly within her chest given that's how some of the romance novels she read would describe. They savored the feeling of their lips pressed against each other for as long as they could until Byleth reluctantly pulled away and removed her hand from Edelgard's chin.

They both opened their eyes and Edelgard no longer saw anger in her former teacher's eyes, but regret. "Perhaps if things were different, in another time, I'd proudly stand next to your side." That was an image Edelgard had seen in her mind countless times. Standing proudly in front of her army with Byleth standing next to her, giving that smile she'd always love. But that would and always will be a dream. A dream she had no choice but to burn away. "Edelgard, let us make a vow right now. Not a vow of love, but of war."

Edelgard couldn't agree more with Byleth's proposition. "Very well, if that's what you wish." She placed her hand on the top of her chest. "I, Edelgard Von Hresvelg, vow to strike down anyone who opposes me. Including you, my teacher."

Byleth did the same and placed her hand on top of her chest. "And I, Byleth Eisner, vow to stop you and your reign of tyranny. I will be the one to kill you and no one else."

Edelgard would have it no other way. "See to it that you survive so that you may reach that moment." She turned away from Byleth. "Professor, no, Byleth. Thank you, for everything." She began to walk away and this time, Byleth did not call out to her. As Edelgard walked down the stairs, she lightly touched her lips and could still feel Byleth's on hers. "So, she did return my feelings after all. Maybe if I told her earlier..." She closed her eyes and shook her head before opening her eyes again. "No, the time for ifs has long passed. She and I are enemies now and I now must burn any lingering emotions for her. As I've told you, Byleth, for when we meet again, one of us shall breathe their last breath."

As soon as Edelgard left, Byleth took her leave as well. Of all the times for her to express how she felt about Edelgard, it had to be this way. There was some regret Byleth felt about proclaiming her vow to Edelgard. Through that kiss she and Edelgard shared, Byleth knew Edelgard shared the same feelings through that kiss, but any chance of romance of love between them was gone the moment Edelgard revealed herself as the Flame Emperor, the person who was responsible for kidnapping Flayn for her blood, the devastation of Remire Village, and the death of the only family she had left, Jeralt Eisner. She couldn't see herself joining Edelgard side after knowing all that. And now here she was. Here they were; two people who had feelings for each other, and yet are sworn enemies. If Byleth wanted even the smallest chance of victory over the new Emperor, she had to let Edelgard know how she felt about her before casting away such feelings. There was no going back to what they were before after this night for either of them.

Now, the feelings Byleth once had for Edelgard were replaced by determination. However, determination could only get one person so far. Hopefully, Seteth was still alive and maybe they could come up with a plan together to stop Edelgard and put an end to her once and for all. "Edelgard, I will stop you. I will not let you conquer all of Fódlan and threaten those that I care about. You will pay for what you did and I will see to it to the very end."