Fire Emblem is the sixth chapter of Tokyo Mirage Sessions ♯FE.
At the Bloom Palace, Tiki reveals to the party a possible method to stopping the Opera of Shadows. Creating a unique Idolasphere called the Area of Memories (named as such for being based on Tiki's memories of the Mirage's original home), she has the party enter in order to gain the right to perform the Opera of Light: Fire Emblem, which can counter the Opera of Shadows by summoning the Divine Dragon Naga. This requires the party to battle the heroes of the past, and upon beating them, gaining roles to play in the Opera. During this, they learn about Gharnef's past, and how he summoned the Shadow Dragon Medeus to destroy the world, only to be defeated by the Hero-King Marth.
Before the party can complete the final trial, which would allow Itsuki to play the role of Marth, Yatsufusa appears alongside Gharnef to claim Marth's soul for themselves. It is then revealed that in order to defeat Medeus, Naga had to remove the world's Performa, hence why Gharnef came to this world, wiping out the Mirage's memories in the process. Despite successfully defeating Gharnef (who Yatsufusa allowed to completely consume his Performa), he still manages to steal Marth's soul in order to complete the Opera of Shadows, resurrecting the Shadow Dragon. Retreating back to the Bloom Palace, the party concludes the only way to perform the Opera of Light now would be to go directly to Medeus, and after explaining the situation to Maiko, Barry, and Ayaha, they head off to the Cosmic Egg.
Traveling through Illusory Dolhr (which includes rematches against Garrick, Aversa, Gangrel, and Excellus), the party arrives at the alter used to perform the Opera of Shadows, which looks similar to a stage. Before the Opera of Light can begin in earnest, however, Medeus suddenly appears and attacks Itsuki, nearly killing him. This prompts the souls of the previous heroes to attack the Shadow Dragon, freeing Marth's soul, who then proceeds to save Itsuki, granting him a new Carnage Form. At this point, Tiki arrives alongside Maiko and Barry, and together the Opera of Light is finally held, ending with Tiki assuming her own dragon form.
After an arduous final battle, Medeus is finally slain, thus saving the world. Itsuki returns to normal, and the staff of Fortuna Entertainment head off to celebrate. As they leave, however, Tiki looks off into the sky where Marth's soul went.