Talos, once known as Tiber Septim, Ysmir, or the Dragonborn, Heir to the Seat of Sundered Kings, is the greatest hero-god (like a demigod) of Mankind, and is worshipped as the protector and patron of just rulership and civil society.
Tiber Septim conquered all of Tamriel and began the Third Era, as well the Third Empire. At his death he ascended to godhood as the God of War and Governance of the Nine Divines, making him the highest of the Nine. Less prominent sects, such as the Talos Cult, were inspired by his apotheosis.
Known as Heir to the Seat of Sundered Kings, Ysmir, the Dragon of the North and Dragonborn, he is also invoked as patron of questing heroes. He is referred to as the One in the phrase “The Eight and the One”, as there were originally only Eight Divines before Tiber Septim ascended to godhood.
In Skyrim, the worship of Talos is banned throughout the Empire as per the agreement of the White-Gold Concordat. This treaty, which brought peace to the land, was formed on the condition that Talos worship would be outlawed. This allowed the Aldmeri Dominion (Whose agents within Skyrim are known as Thalmor) to stamp out Talos worship and worshippers throughout the Empire.