Dragon Crisis

The Dragon Crisis was a time when Alduin emerged from the time rift he was banished in during the Dragon War. He caused destruction and chaos throughout Skyrim. He also revived many dragons in an attempt to restore the rule of Dragons over Men. However, the Last Dovahkiin rose and was able to defeat him as foretold by the Elder Scrolls.

The main questline of prominently features the Dragon Crisis.

Background
During the time of the human rebellion of the Dragon rulers, three heroes were able to defeat Alduin using an Elder Scroll. However, they did not completely destroy him. But rather send him in a time rift through the Elder Scrolls.

Battle at Helgen
Alduin exited the time rift during the Civil War. The time where the Stormcloaks and the Imperials for fighting over Skyrim, Dovahkiin was attempting to cross the border of Cyrodiil, only to be captured in a Imperial trap. They captured many Stormcloaks, including their leader, Ulfric Stormcloak and a horse Thief. As they are taken to execution, a Dragon attacks. Alduin, Ender of Times. He provided enough of a distraction to let the Dovahkiin escape, as he escapes out of Helgen and into Riverwood.

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 Mirmulnir at the Western Watchtower during the Battle of the Western Watchtower. This was the time where he/she figured out he/she was a Dragonborn, before becoming Thane of Whiterun. The Dovahkiin then went to the Greybeards for training after the Greybeards shouted his name as a call.

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 Sky Haven 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 acquired the Elder Scroll the Dovahkiin traveled back to the Throat of the World and discovered how Alduin managed 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 by using the mortal-made shout Dragonrend, the exact same shout used to defeat him long ago. 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, Alduins right hand. After the Dragon's capture, he confesses that some Dragons started doubting that Alduin is truly the strongest among them, he then strikes a deal with the Dovahkiin, which is to free him and then he takes him to the gate to Sovngarde. They rode this dragon to Skuldafn, after Dovahkiin defeats the Dragon Priest guarding the portal, he/she is transported to Sovangarde. Alduin can be seen consuming souls of the dead, Dovahkiin searches for the three great warriors who defeated Alduin long ago, but first, he defeats Tsun, who allows him to enter. As the warriors and the Dovahkiin unite, they battle Alduin in the Battle of the Tongues.

Aftermath
In the aftermath of Alduin's defeat, the rest of the dragons now recognized the Dragonborn as superior to them and agree to cease their hostilities, acknowledging the power of his Thu'um and his defeat of Alduin.

However, if the Dragonborn did not kill Paarthurnax, the Dovah will now acnknowledge him as their King, with Paarthurnax stating his intention to bring The Way of the Voice to all Dragons.

In either case, however, some Dragons still act to terrorize Nirn.