Seiros was 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.
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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, one of the "Children of the Goddess" who lived in Zanado, the Red Canyon, before Sothis was betrayed and murdered by Nemesis, and her home razed by his forces. Traumatized and embittered, Seiros became distrustful and vengeful towards humans, as she blamed Nemesis's human nature for becoming corrupted by power. She became obsessed with reviving Sothis and returning the happiness of her childhood.
The War of Heroes was motivated by her vengeance against Nemesis. She 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, she led the battle against Nemesis and the Ten Elites, killing Nemesis in Imperial Year 91 at the Tailtean Plains. After the War of Heroes, she 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, and obscuring Nemesis' original past, only leaving intact that he was a hero who fell from grace.
After the establishment of the Church, Seiros's ultimate goal was to revive her mother, using the Church to control Fódlan from the shadows, eliminating any who would oppose the Church, and using the Crest system to find a way to bring back her mother. After her "death", she would continue to keep control over the Church by assuming the identity of Rhea. As Rhea had shared her blood with Jeralt to save his life and used the Crest of Flames to revive a newly born Byleth, Seiros believes them to be the perfect vessel for the Goddess.
Seiros may be derived from Seirios, Greek for "scorching", and an alternate name for the star Sirius.