Akatosh is the chief diety of the Nine Divine. He takes the form of a dragon and is said to have appeared in the beginning and imposed his regulation of existence upon the universe, Void, Oblivion, Mundus, Aetherius, and outer realms.
When Lorkhan tricked other et'Ada into giving up themselves or parts of themselves to create Mundus (universe and Nirn (the world) and its moons) Magnus saw the trap and ripped a hole in the sky as he fled to Aetherius, creating the sun and his followers joined him creating stars as they tore through the universe. Akatosh saw the trap as well but did not flee and instead threw himself into Mundus. He landed in what became known as the Isle of Balfiera in High Rock. His full presence caused all spirits and chaotic forms of creation to crystallize and time to become linear and ordered.
He embodies the qualities of endurance, invincibility, and everlasting legitimacy while promoting the virtues of duty, service, and obedience. Akatosh’s blood was mystically joined with Alessia and her heirs forging the Amulet of Kings which maintained a barrier between the mortal realm and the planes of Oblivion.
During the Oblivion crisis when the barrier was broken with the assassination of Uriel Septim VII and his heirs and gates to Oblivion opened all over tamriel, Mehrunes Dagon was able to briefly enter the mortal realm. Martin Septim the last and illegitimate heir of Uriel Septim VII shattered the Amulet of Kings and was able to harness the spirit of Akatosh and become his avatar in the form of a bipedal flaming dragon. After pushing Mehrunes Dagon back into Oblivion the avatar of Akatosh turned to stone, killing Martin and remains standing amongst it ruins of where the Temple of the One once stood.