Alduin

Alduin: The world-eater who's return will mean the enslavement of humans and the end of the world

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Alduin, was the first dragon among many that ruled over men and mer. The dragons viewed humanity as a lesser species, and kept a tyrannical hold on mankind, but soon, in an event known as The Dragon War, men stood up to their dragon overlords, and learned how to use the Voice (Dragon Language). On the Throat of the World, Alduin was defeated, but not killed. Instead, with an Elder Scroll, Alduin was sent forth in time, albeit unintentionally, and would not return until the 201st hundred of the fourth era.

After the Delphine, the last of the Blades discovered the Dovahkiin of the Fourth Era, she helped them discover from Paarthunax that the key to Alduin's defeat was the Dragonrend Shout Septimus Signa, an expert on Elder Scrolls, helped the Dragonborn locate an Elder Scroll in Alftrand, which the Dragonborn used atop the Throat of the World to view the memory of Alduin's initial defeat. Attracted by the power of the scroll, Alduin appeared at the site. Together with Paarthanax and the their followers, the Dragonborn weakened Alduin, who eventually fled the battle.

The Dragonborn finally kills Alduin in Sovngarde with three other heroes of Sovngarde, Hakon One-Eye, Felldir the Old, and Gormaliath Golden-Hilt. Thus, Alduin is banished from the world forever. (From Elder Scrolls Wiki)