Fire Emblem: The Sacred Stone (Nintendo Cinematic Universe)

Fire Emblem: The Sacred Stones is a Japanese graphic novel.

Summary
When the Grado Empire of the five nations of Magvel invades its neighbors and begins destroying the stones, Eirika and Ephraim of the royal family of Renais set out to gather allies and halt Grado's conquest, while also trying to uncover the reasons for the war.

Plot Elements

 * Long ago, the five nations of Magvel were each given charge of one of five magical gemstones called the Sacred Stones. The Stones were used to seal away the soul of the Demon King Fomortiis at the end of a conflict between humans and monsters over 800 years ago.
 * The Grado Empire, largest of the five founding nations, launches an unexpected assault on the neighboring nation of Renais, home of the royal siblings Eirika and Ephraim.
 * Taken by surprise, Renais is defeated and the two siblings are separated: Eirika escapes to the northern nation of Frelia with the king's general Seth, while Ephraim goes underground to mount a resistance against the Grado Empire to the south.
 * As Eirika seeks aid from the other nations and gathers allies, the land becomes plagued with undead monsters, a sign of Fomortiis's return. She is eventually reunited with Ephraim, and the two discover that Grado plans to destroy the Sacred Stones and unleash Fomortiis once more.
 * After failed attempts to contact Grado's Prince Lyon, the twins' closest friend, they are forced to mobilize against their former ally as the Empire turns its attention to the other nations' Sacred Stones, successfully destroying the Stone of Frelia.
 * The siblings part ways once again, each on a mission to preserve the Stones.
 * Eirika is forced to reach the allied nation of Rausten by passing through the new republic of Carcino, the sixth nation, and secret ally of Grado. Her forces escape Carcino by passing through the desert nation of Jehanna, where the Stone of Jehanna is destroyed and Eirika is cornered by two legions of Grado's army.
 * Meanwhile, Ephraim is joined by allies from the nations of Frelia and within Grado itself, and they fight their way into the heart of the empire, to the very throne of Grado's emperor, Vigarde.
 * After defeating Vigarde in battle, Prince Lyon appears and reveals that the emperor was merely a resurrected puppet, and that he had started the war. When Vigarde died of illness months before, Lyon had feared his inability to rule Grado and became desperate to bring his father back. Lyon sacrificed his nation's Sacred Stone to revive the Emperor's body, but inadvertently absorbed the fragment of the Demon King's shattered soul sealed within. Lyon became a slave to the Demon King's will, and Vigarde a soulless puppet. Lyon declares that he will once more rule Magvel once the Stones are destroyed. With this revealed, he vanishes. Burdened with this knowledge, Ephraim heads to Jehanna and rescues Eirika.
 * Once reunited, the siblings gather their forces and liberate Renais, retrieving its true Sacred Stone, which had been replaced by a decoy when Grado invaded.
 * After gathering reinforcements from Frelia, the twins head east to Rausten, hoping to protect its Sacred Stone. On the way, the twins are confronted by Lyon, who bests the twins and finally destroys Renais' true Sacred Stone.
 * The twins manage to escape and retrieve the last intact Sacred Stone from Rausten.
 * The siblings pursue Lyon to the Darkling Woods, where he is performing a ritual which will resurrect the Demon King.
 * The twins do battle with Lyon and finally kill their former friend, inadvertently supplying him as the final sacrifice that resurrects the Demon King.
 * Eirika and Ephraim use the Sacred Stone of Rausten to seal the Demon King's soul, and then their forces destroy its body.
 * The siblings' allies from the other nations return home, while they themselves seal the final Sacred Stone away and set about restoring their nation.