Nemesis (Fire Emblem)

Nemesis, also known as the King of Libration and the Fell King, is the overarching antagonist of Fire Emblem: Three Houses and the archenemy of Seiros. He is also behind the creation of the Crests and the conflict on Sothis.

He initially serves as a historical figure for most of the game, but in the Golden Deer route, particularly in the Verdant Wind chapter, Nemesis serves as a much more important figure when he becomes the route's true main antagonist and its final boss after his unexpected revival by the forces of Those Who Slither in the Dark.

He is voiced by Naomi Kusumi in Japanese version and by Jamieson Price in English version, with the latter Rudolf from Echoes: Shadows of Valentia, and Zephiel and Hawkeye in Fire Emblem Heroes.

Past
According to the historical legends by the Church of Seiros, Nemesis was once a king who the Goddess Sothis entrusted him to protect Fódlan from hostile invaders. However, over time, he gone corrupted and grew more power hungry after being influenced by the people of Agartha, deserting his duty and becoming a bandit king. He worked with them to slaughter the "Children of the Goddess" in Zanado, and was responsible for the death of Sothis via killing her in her slumber to use her body parts as weapons.

Using Agarthan technology, Nemesis and his allies, the Ten Elites, created the Crests from the hearts of the people of Zanado and the Heroes' Relics from the bones of the Goddess, distributing them to Ten Elites.

The truth of the legend, however, is that Nemesis was never more than a common thief who managed to break into the Holy Tomb, stealing Sothis' body. Those Who Slither in the Dark then used the blood of Sothis to give him the Crest of Flames and the body of Sothis to make the Sword of the Creator.

In Imperial Year 91, the armies of Nemesis and Seiros clashed at the Tailtean Plains, where Nemesis was confronted and killed by Seiros in single-combat.

Verdant Wind
After the war of Shambala, the leadership of Those Who Slither in the Dark was eliminated by Byleth, Rhea, Claude and the army of Leicester Alliance, but turned out Those Who Slither in the Dark members already succeeded in reviving Nemesis, whom their remnants manipulated into invading Fodlan and destroying everyone in his path along with the Ten Elites. After a long time of struggle, Byleth and Claude worked together and finally slain Nemesis for the second and final time in battle.

Trivia

 * The tale of Nemesis being a bandit king is disputed by Edelgard, who claims she was taught Nemesis was not power hungry, and instead his war with Seiros was a simple dispute over what type of being should rule over Fodlan, humans or creatures who can take human form.
 * Etymology: Nemesis is named after who enacts retribution against those who succumb to hubris (arrogance before the gods).
 * Another name for her is Adrasteia (or Adrestia), meaning "the inescapable". It became the namesake of Adrestian Empire.