Flame Emperor

"I'll let them roam free for a bit longer. I don't have enough information about that teacher."

- Flame Emerpor talking about Byleth

"Such a brutal, irrational world we live in. Some believe the Crests, tokens of the Goddess's power, are necessary to maintain order. But they're wrong, Teacher. The Crests are to blame."

- Edelgard speaking with Byleth

Flame Emperor is a major antagonist in Fire Emblem: Three Houses and the mysterious leader of the Flame Emperor Army. They first appear as a mysterious figure who work reluctantly with Those Who Slither in the Dark against the Church of Seiros, but is shown to have their own agenda.

In all the routes before the climax, the Flame Emperor is actually revealed to be Edelgard von Hresvelg, one of the main protagonists of Fire Emblem: Three Houses and the princess and future Empress of Adrestian Empire. There are two critical choices the player might make in the Black Eagle route will decide her fate, either continuing the Black Eagle route or transfer to the Church of Seiros route automatically.

Post-timeskip, Edelgard becomes the Empress of Adrestian Empire and will become the deuteragonist in the Black Eagle route, a major antagonist in both Church and Golden Deer route, and the main antagonist in the Blue Lion route. Only in Black Eagle route, Edelgard will survive and redeem herself from her extreme tendency.

In Japanese version, she is voiced by Ai Kakuma. In English version, she was voiced by Cristina Vee in E3 2018 trailer and later by Tara Platt in the completed game.

Personality
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Backstory
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Church Route & Golden Deer Route
In both the Church of Seiros route and the Golden Deer route, Edelgard's roles are relatively the same. In the first part, he Black Eagle route will switch into the Church of Seiros route in Part 2 if two conditions are made: Five years later, Byleth and Edelgard will meet in her palace for a final confrontation. After her defeat, Edelgard shows remorse over her actions and pleadge Byleth to execute her. In her last moment, Edelgard bemoans how she once wished to be side by side with her former mentor, before Byleth begrudgingly beheaded her.
 * 1) Byleth chooses not to speak with Edelgard before her coronation, or rejects her offer to take part in her conronation.
 * 2) If Byleth chooses to speak with Edelgard and participate her coronation, there will be an extra alternative choice in the laybrinth when Rhea orders the execution of Edelgard, now revealed to be the Flame Emperor. If Byleth chooses to execute Edelgard, Hubert will protect Edelgard and escape, leaving Byleth's party permenantly.

Trivia

 * Thoughout the entire game, at least one of Edelgard and Dimitri would die if the chosen route is not their own. It is impossible to make both of them survive.
 * There are certain foreshadowing that Edelgard is the Flame Emperor. For example, both Edelgard and the Flame Emperor wears red, and Edelgard holds the title of the future Empress of Adrestian Empire. Edelgard will also be absent once Byleth faces the Flame Emperor face to face, and will appear once more . In addition, Edelgard is skilled in using axes, and her axe used in Fire Emblem Heroes is the same axe used by the Flame Emperor.
 * Edelgard can be romanced by either gender of Byleth once they reach S-Support, making her one of the few bisexual playable characters in the game.
 * Initially in the English version, Edelgard was voiced by Cristina Vee in the original E3 2018 trailer for Three Houses, whom she shares with Tatiana from Fire Emblem Echoes: Shadows of Valentia, as well as the Fire Emblem Heroes appearances of Lachesis and Athena.