Their origins of this rebel group date back to the White-Gold Concordat, which banned the worship of Talos. As a means to make sure these laws are regulated, the Thalmor track down any Talos worshippers and imprisoned them. The Nords, who often revered the what they believe as the Ninth Divine, found this law to be outrageous, and as a result start rebelling against the Empire. The death of High King Torygg at the hands of Ulfric Stormcloak in 4E 201 was the main spark of the Civil War. Alternatively, the Dragonborn can side with the Imperials and bring an end to the rebellion.