The true biggest flaw of Fe3h is Fódlan doesn't have a Halloween. Can you tell me Hubert and Bernadetta wouldn't go all out for Halloween?
The idea for the event comes about during the White Heron Cup, where Bernie expresses that the dance would be a lot less scary if she could hide her face. She and Hubert had just undergone their B support and Hubert is still looking for ways to make Bernie comfortable around him while not sacrificing his own self-expression. Event Planner Hubert, who secretly enjoys parties but wishes they weren't filled with pretention, invites everyone to a "mysterious costume party" and frame it as a masquerade ball. (We're ignoring the fact that in canon he's like "really, the archbishop wants to throw a party right now?"). With the Vestra planning skills passed down for generations, he takes over the dining hall one night and books Manuela and Dorothea as entertainment (Ferdinand also joins, because he will take any and all opportunity to sing). Ideally, he decorates with as many skulls as he could possibly accumulate (”where did this come from?” “*evil laugh* wouldn’t you like to know”) and many candles. Even for Hubert, however, skulls are hard to come by, so Bernie ends up carving faces into pumpkins. Her weird idea is met with delight. The whole scene is slick and dark and the dance itself is held at midnight.
It 100% gets shut down in the middle, but it sure as hell was worth it.
What Everyone in Black Eagles Wears:
Edelgard - *slams fist on table* valkyrie Edelgard!!! She and Hubert make a set of black wings with leftover feathers from the Flame Emperor costume and she uses her armour from class as a template. She is many girls' gay awakening that night.
Hubert - a Dark Falcon Knight costume complete with a black helmet with plummage. It covers his eyes sharply, in red, and his cape trails behind him on the ground.The black feather details are kinda sus, but people go "oh it's just Hubert being dramatic". He's just too obvious to be involved in the Flame Emperor plot, come on now . . .
Bernadetta - of course I'm putting Bernie-bear in a bear costume. She has an adorable brown cloak with little bear ears on top that she made herself. The hood comes down really low, so much so that she put slits for her eyes. After the event she starts attending class like that. Byleth wants to make it the house uniform on the battlefield.
Dorothea - she borrows one of Manuela's old costumes, from one of her starring roles as a showgirl who marries into nobility. There are full peacock feathers circling her and there’s more jewels on it than she’d ever worn in her life. Manuela’s performance as this role was her bi awakening and wearing the costume feels too sacred (”if I sneeze in this outfit I will destroy it and owe the opera money even though the production is long over and Manuela owns it”). Petra notes that Dorothea is reminding her of home, because she looks like an exotic bird. She takes breaks from singing to flirt with the Bleagles girls mingle!
Ferdinand - Dorothea tells him he'll be in her good graces if he comes dressed as a bee. Not only does he oblige, but he goes all out. He wears a black jacket with sharp tailcoats to emulate a stinger, a black-and-yellow striped shirt, a gold vest, and a set of wings. He wears a bee antennae headpiece with flowers attached to it. Dorothea feels a little bad for making him go through the effort but he is genuinely embracing the bee lifestyle and everyone is super impressed with him.
Caspar - he just wears his battlefield clothes. (I made him a grappler in my first playthrough, so I’m imagining him in that beautiful outfit). When asked if he dressed up, he claims that he’s a warrior.
Linhardt - draws whiskers on his face and claims he's a cat. Everyone teases him for being low effort until Byleth walks in, also with cat whisker face paint and no other effort.
Petra - she initially panics because she wants to attend but has nothing suitable. She doesn’t want to wear any of her Brigidian clothes because they are not costumes. Inspiration hits her one day when Byleth accidentally hits her with their trailing jacket sleeves while zooming through the monastery at a speed no one goes at in their everyday life. She borrows a jacket and slumps it over her shoulders, puts her hair down, and tries to keep her expression blank. She even makes a copy of the Sword of the Creator. The costume is a massive hit.
Additionally, consider the following interaction:
Edelgard: It's no use. Nothing I own is suitable for the occassion!
Hubert: Well, save for that one costume.
Edelgard: . . . I am not exposing myself as the Flame Emperor for a party