Seiros

"So, this is the choice you have made. You are just another failure. [...] I... have passed judgment, and now I shall rip your chest open, and TAKE BACK YOUR HEART MYSELF!"

- Rhea's Breakdown upon seeing Byleth protecting Edelgard (Crimson Flower route)

"I have no patience for foolish questions. I shall sacrifice as many lives as it takes! That apostate who insists on taking everything from me WILL BE CRUSHED BY MY OWN HANDS!!!"

- Rhea/Seiros after turning insane (Crimson Flower route)

Seiros, also known as Rhea (her modern incarnation) and the Immaculate One, is the deuteragonist and one of the two secondary antagonists (alongside the Flame Emperor) in Fire Emblem: Three Heroes, serving as the founder, patron deity and Archbishop of the Church of Seiros.

She serves as the deuteragonist in the Acadamy Phase (as Rhea) pre-timeskip. In the War Phase post-timeskip, she serves as either an alluded character a supporting character in most of the routes, being the deuteragonist (and the final boss after she gone berserk) in the Church route, a mentioned character in the Blue Lion route, and a major character in the Golden Deer route.

However, in the Black Eagle routem, Rhea gradually descends into madness if Byleth choose to shield Edelgard, who was revealed to be the Flame Emperor. She becomes fully controlled by Seiros' dark personality and becomes the main antagonist in Crimson Flower chapter afterwards.

She is voiced by Kikuko Inoue in Japanese version and by Cherami Leigh in English version.

NOTE: This page is only for Rhea/Seiros in the Black Eagle route, particularly its Crimson Flower segment, which is the only route where she serves as a villain.

Origin
According to the Church's myth, Seiros is the founder of the Church of Seiros. It is said she received a revelation from the Goddess that was a "gift to guide the lost." 1,000 Years before the start of Three Houses, Seiros fought against the King of Liberation Nemesis in the War of Heroes.

However, the myth of the Church is a deliberate partial distortion of real history. Seiros was not a normal woman, but was the daughter of the Goddess Sothis, being one of the "Children of the Goddess" who lived in Zanado, the Red Canyon, before Sothis was betrayed and murdered by Nemesis, a former guardian of Seiros who became a bandit after being corrupted by his desire of power.

Traumatized and embittered, Seiros became distrustful and vengeful towards humans, blaming Nemesis' hunger for power and subsquent treachery for a consequence out of human nature. She became obsessed with reviving Sothis and returning the the happiness of her childhood.

War of Heroes
To exact her revenge against Nemesis, in Imperial Year 91, Seiros formed an alliance with one of Nemesis' allies, Wilhelm I, by sharing her blood with him, giving him the Crest of Seiros. With the combined forces of herself, Wilhelm and the survivors of the Red Canyon, the Four Saints, Seiros led the battle against Nemesis and the Ten Elites, killing him in a hand-in-hand combat at the Tailtean Plains.

After the War of Heroes, Seiros founded the Church and rewrote history, claiming that the Crests were gifts from the Goddess to humanity, that the Ten Elites were her devout followers, obscuring Nemesis' original past and only leaving intact that he was a hero who fell from grace.

After the establishment of the Church, Seiros began her endeavor to revive Sothis, using the Church to control Fódlan from the shadows. She eliminated any who would oppose the Church and using the Crest system to find a way to bring back her mother.

Reincarnation
After her eventual death, Seiros would continue to keep control over the Church by possessing Rhea, her reincarnation, causing Rhea to develop a gradually darker and more bloodthirsty personality as Seiros takes control. As Rhea had shared her blood with Jeralt to save his life, she also used the Crest of Flames to revive a newly born Byleth. Seiros believed that Byleth would be the perfect vessel for the Goddess.

However, Jeralt grew suspicious of Rhea due to the circumstances of his wife's death and Byleth's condition, who had a pulse but no heartbeat. Losing his faith in Rhea after believing his wife died of something else, Jeralt faked the death of him and his child and left the Knights of Seiros, secretly smuggling Byleth with him.

Black Eagle route (Crimson Flower)
Note: This route can only be achieved by choosing the Black Eagle, attending Edelgard's ascension ceremony in time and choosing to spare her.

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Trivia

 * Seiros/Rhea is the only final boss of the game that is fought in two routes.
 * Etymology: Rhea is named after a Greek titan known as the "Mother of the Gods" due to being the mother of major Olympians including Zeus, Poseidon, Hades and Hera. The name may also refer to the mother of Romulus and Remus, the mythical founders of Rome.
 * Seiros may be derived from Seirios, Greek for "scorching", and an alternate name for the star Sirius.