Ozai was the Fire Lord and absolute ruler of the Fire Nation during the final years of the Hundred Year War. He was the second son of Azulon and Ilah, the younger brother of Iroh, uncle of Lu Ten, husband to Ursa, the father of Prince Zuko and Princess Azula, and paternal grandfather of Izumi. Ozai was a firebending master and, prior to the ending of the War, was believed to have been one of the most powerful firebenders in the world.
After Iroh's only son, Lu Ten, died, Ozai usurped his brother's birthright to the throne in a scheme that resulted in his father's death and Ursa's banishment.
Ozai's rule saw a mass buildup of industry and significant technological advancements. Like his father and grandfather before him, Ozai sought to conquer the other nations and become the supreme ruler of the world. To do this, he planned to utilize Sozin's Comet to burn down the Earth Kingdom, renouncing the title of Fire Lord and declaring himself the Phoenix King, ruler of the world. Before his plans could come to fruition, he was defeated by Avatar Aang, who used energybending to remove his firebending abilities permanently.
After the end of the war and Zuko's crowning, Ozai was sentenced to life in prison for several war crimes. However, he was able to maintain a degree of influence on world politics through some of his loyal followers creating the "New Ozai Society" and his son, who occasionally sought advice from Ozai. As a result, the former Fire Lord was indirectly responsible for the failure of the Harmony Restoration Movement.