Dragon Crisis

The Dragon Crisis was a time in Skyrim when the World-Eater Alduin rose from Sovngarde after his first defeat and started resurrecting dragons. Unfortuneatly for him, this is also the time when the Dragonborn rose and was able to defeat him.

Backround
During the time of the human rebellion of the Dragon rulers, three heros were able to defeat Alduin using an Elder Scroll. This prompted the dragon to return to Sovengard and regain his strength. During his rising, he unknowingly liberated the Dragonborn from Helgen and, through time, was able to defeat Alduin.