Dragon Crisis

The Dragon Crisis was a time in Skyrim when the World-Eater Alduin was flung into the Fourth Era after ancient Nord heroes cast him through a time rift by use of an Elder Scroll. Unfortunately for him, this is also the time when the Dovahkiin rose and was able to defeat him.

Background
During the time of the human rebellion of the Dragon rulers, three heros were able to defeat Alduin using an Elder Scroll.

Battle at Helgen
Alduin exited the time rift during the Civil War. He attacked Helgen just as Dovahkiin was about to be executed for crossing the border into Skyrim. Through this he saved not only the Dovahkiin, but also Ulfric Stormcloak, allowing the civil war to continue.

Death of the First Dragon
The Dovahkiin escaped from Helgen and eventually ended up in Whiterun. They went with the Jarl's housecarl and some guards and defeated Murmulnir at the Western Watchtower during the Battle of the Western Watchtower. This was the time where they figured out they were Dragonborn, before becoming Thane of Whiterun. They then went to the Greybeards for training.

Hunt for the Elder Scroll
After training the Dovahkiin found the last remaining members of the blades Esbern and Delphine. After finding Alduin's Wall in the Skyhaven temple they returned to the Greybeards, and met with their leader, the dragon Paarthurnax. Paarthurnax sends the Dovahkiin to find an elder scroll.

The First Battle With Alduin
Once the Dovahkiin had aquired the Elder Scroll the Dovahkiin traveled back to the Throat of the World and discovered how Alduin manged to come back. After the Dovahkiin looks into the Elder Scroll, he fights Alduin with another dragon, Paarthurnax. Together they nearly defeat Alduin during the Duel Atop the World, but he escapes back to Sovngarde.

The Final Battle
The Dovahkiin convinces the Jarl of Whiterun to allow them to use Dragonsreach as a prison for Odahviing. They then ride this dragon to the portal of Sovngarde. After defeating a dragon priest the Dragonborn goes to Sovngarde and, teaming up with some of the Heroes of Sovngarde, they defeat Alduin, once and for all, during the Battle of the Tongues.