Instead of falling off that cliff and going missing for five years and waking up in a river, Byleth falls down and somehow survives but is found by Imperial Troops while they’re knocked out.
In my own play through of the Blue Lions Route I fought Ferdinand and he said that Edelgard has always been obsessed with Byleth, which means she is fixated on the professor even outside of having them as her homeroom teacher.Â
So Edelgard high-key has a “Capture If Possible” order. She’s not confident that they can, because the professor is strong and Edelgard doesn’t doubt that they’ll have to die in order for her plans to come into fruition.Â
But, by some twist of fate, the Imperials find Byleth beneath the ruble, unconscious but alive. The Sword of the Creator is gone, but it’s easy for them to capture Byleth like this and transfer them to the Imperial Capital.Â
Edelgard is...ecstatic. Surely, this is proof that her destiny is right. One of the things she wanted most was the professor by her side and now fate has gifted her with them. She knows they’ll resist, try to get back to the Faerghus and the Blue Lions, but in time, once she’s won, perhaps her teacher will see that her path is the best path.
They keep Byleth unconscious the whole way back to Ennbar, drugging them and making sure they don’t wake. No one wants to fight them, even without a weapon. They could kill Byleth, perhaps, because Byleth is unarmed and outnumbered, but it would make the Emperor very unhappy, so it’s easier to keep them drugged until they have a prison set up.
Edelgard has the most elaborate, but comfortable, prison constructed. There’s only one door, a solid metal construction with multiple locks and bolts, and there are no windows. It’s built in a high tower, with winding stair cases and halls that lead nowhere, a veritable maze just in case. The cell itself looks more like a guest room than a cell. It has a nice bed, more lavish than Byleth’s bed at Garreg Mach, and plenty of books to keep them from getting bored, and silk clothing (red, it’s all red, Edelgard’s colors). If you ignore the fact that Byleth won’t be allowed to leave when they wake up, you could almost forget they’re a prisoner. She has only her most trusted friends (the Black Eagles) and servants enter this tower, and the fact Byleth is even being kept there is very hush hush for the most part.
Those Who Slither In The Dark try to get access to Byleth, but Edelgard isn’t having any of it. She’ll let them have Rhea, but they are NOT touching the professor. It’s the cause of a lot of tension between the two factions.
Meanwhile Byleth is only being visited by the best healers in Edelgard’s employee/confidence. It takes a long time, but Byleth wakes up much earlier because of it.
Byleth...isn’t happy to be here.
Byleth was the head of the Blue Lion House, and they’re loyal to the Blue Lions and Dimitri, and the last they saw of Edelgard she was the Flame Emperor, who was involved with the ones who killed their father, and probably caused the Tragedy of Duscar, etc. They’re not happy at all. They’re worried sick, and anxious, because Dimitri wasn’t looking so good before the battle, and they don’t know where their students are, and they want to know what’s going on.
They refuse to eat when the Black Eagle students bring them food. They sit there, silently glaring at them, demanding to know why they’re here and what they want. Their former students assure them that they’re not going to be hurt, that they’re just here for their own safety until the war is over, but Byleth is in a less than gracious mood.
They don’t attack, because they’re a smart and they’re aware that even if they got out of the room there’s probably a whole slew of guards, and they don’t know where they are. But refuse to eat, no matter how much the students beg them to. No amount of arguing, or insulting, or begging will get them to lay a hand on the food. It may seem like a stupid move, with their body growing weaker and weaker, but Byleth doesn’t have very many other ways to fight back right now.
Eventually Edelgard herself comes to deliver food in a personal attempt to get Byleth to eat. They don’t, at first, simply demanding to know what’s going on, and where Rhea is, and Dimitri, and pretty much any news from the outside because no one has been telling them anything.
 Edelgard is harsh on them. She bargains information for their cooperation eating food. She’s brutally honest about her answers too, Rhea is captured, she’s invading the rest of Fodlan, Dimitri hasn’t been seen since he murdered his uncle (she says that specifically to make his less desirable than her, because why should the professor worry about a half mad man like that?), Claude is barely holding the alliance together, and she’s confident in her victory.
Byleth refuses to eat in Edelgard’s presence again. They refuse to speak or even LOOK at Edelgard. At first, Edelgard is hurt and furious, arguing with them about why she’s right, how much crests have hurt her, how opening up like she would have if they were her homeroom teacher. But it’s like yelling at a wall.
The truth is, Byleth does have sympathy for Edelgard, but they can never condone this war, and they can’t stand her allies that killed their father. And even if they could somehow get past those two thing, in their heart they know they’ll always be haunted by Dimitri’s hurt, betrayed, face when that mask fell off.Â
Dimitri was the one that helped them learn how to be human, who came to comfort them with kind words and understanding when their father died, who slowly and honestly opened up about his past, who understood and included Byleth even when he was intimidated by their expressionless face, and they don’t think they’ll ever quite let go of that loyalty.
So Byleth plans their escape.
It takes a long, long, time. But they start to eat more when certain students showed up with food, strategically stop asking certain students certain questions, start asking different ones to others. They memorize how long time passes between visits, only to find it erratic. They don’t give up though. They learn to tell time in the guard’s footsteps that sound outside their door. They count how many are taken with each shift change, and how long they can count before the next shift, and what student will come in between those shifts in case there is any pattern. They count how many steps are taken before the sounds of footsteps disappear, and mentally map out the first floor from there.
It’s just pure bad luck that Bernadetta happens to be the student visiting when Byleth enacts their escape plan. They knock her out and run for it, stealing any money and gold she might have on her (not much, just pocket change, because why would she need it if she’s just going to feed her teacher?). They knock out one of the guards before he can make a noise, and take out the rest in sneak attacks before they can sound an alarm. Once the first floor is cleared, then comes the hard part.
They steal a guard’s armor and clothes, locking them in the room with a still knocked out Beradetta. Neither should wake for a while anyway, and Byleth locks them in the room and waits for the shift change. Once the next guard is there, they leave.
It’s easier sneaking out of the city than they thought it would be, they just walk out in Imperial Armor.Â
The alarms aren’t sounded until after Byleth has made it well outside the gates, from there it’s a mad dash. They might steal a horse. Luckily, they have an advantage, they’re still in disguise, and they’ve already put distance between them and the search parties. So long as they don’t mess this up, they can pretend they’re searching if they get caught.
It’s a stupid plan, kinda, because they’ll probably be looking for a lone guard, but it’s all they’ve got for now.Â
They don’t stop moving at all, they have to put distance between themselves and Ennbar. At some point their horse flat out dies from exhaustion, and they shed their armor to gain more speed and distance.
They’re almost caught, but they jump into a rapid river and escape. No one is mad enough to jump after them.
They’re pulled out of the river what must feel like days later by a friendly civilian. From them they learn that they’ve been imprisoned for five years.
Five whole years in that damn room.
Somehow, by some twist of fate, this village just so happens to be just outside of Garreg Mach, and the Sword of the Creator washed up beside them. And Byleth can’t help but think that there’s some twisted sense of humor to fate.
Five years, almost to the day.
They wonder if their students have come, the Blue Lions they mean.They wonder if any are alive to keep their promise. Edelgard said no one had seen Dimitri in a long time, and with the war it seems almost silly that any of the others would remember their innocent promise.
Byleth goes to Garreg Mach anyway.