In the Plains of Hoshido, set in another world beyond Smash Bros., A young prince named Corrin witnessed his two opposing families about to fight. Corrin was born in the Hoshido Family with his siblings, Ryoma, Hinoka, Takumi, and Sakura. He was raised/adopted by the Nohr Family with his siblings, Xander, Camilla, Leo, and Elise. Corrin could decide who to aid in battle. Each family tries to plead with Corrin.
Xander: This way.
Sakura: Big brother...?
Elise: He's my brother!!
Ryoma: We're your family.
Corrin just wanted to avoid all this conflict, so he thought it was just best to leave and think this over.
Corrin: I'm... so sorry everyone. I have no other choice but to...
He ascended into his dragon form and left. He went to the Coliseum and saw other the Fire Emblem characters training for the final battle.
He recognized everybody as he read books about them.
Corrin: Hey, excuse me, could I practice with you guys.
Marth: Who are you?
Roy: What doing you here?
Ike: What do you want?
Robin: State your purpose.
Lucina: Can you fight?
Corrin: I am Corrin. I am the Heir to the First Dragons. I am a fan of yours. You're the Hero King, the Young Lion, the Radiant Hero of Legend, the Tactician Magician, and the Warrior of the Doomed Future.
They were all shocked.
Marth: Well we are flattered.
Ike: Let's see what you got.
Corrin was running towards Ike and dodged his Down Smash by jumping and a forward aerial to a side tilt. Then, Corrin's siblings communicated with him telepathically.
Ryoma: What are you doing?! Corrin!
Corrin: I'm sorry, big brother. (While spotdodging Roy's Dash Attack) (performing a down tilt to up aerial) I've made my decision.
Lucina: Who's he talking to?
Marth, Robin, and Lucina prepared themselves.
Xander: How could you? Your giving up on us and battling completely unknown opponents.
Corrin blocked Marth's attack and performed his up aerial, then performing his dash attack on Robin and Lucina.
Sakura: They look tough.
Elise: Not a problem for my big brother.
Corrin blocked Ike's Quick Draw with a Side Tilt, blocked Roy's Down Aerial with a Up Tilt, but fell victim to Ike's Up Smash. Corrin ran up to Robin and spotdodged his Up Smash followed by Corrin's Side Tilt. He jumped over Lucina's Dash Attack, following Corrin's Down Tilt. Corrin's Yato ascended to its Omega Yato form.
Ryoma: The Yato Blade is taking on a form I've never seen.
Elise: Ooh, that looks painful.
Corrin pummeled Marth and performed his Forward Smash on Ike and Robin, tapping into Dragon Fang.
Ryoma: He's using Dragon Fang!
Xander: The power of the First Dragons.
Corrin used Dragon Fang Shot on Roy, Dragon Lunge on Lucina, another Dragon Lunge with a backward followup on Marth. Soon, the other Fire Emblem heroes stood beside one another planning to take on Corrin all at once.
