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.
About eight years before Avatar Aang was found in the iceberg, Ozai's older brother Iroh began the six-hundred-day Siege of Ba Sing Se, the Earth Kingdom capital, which ended when Iroh's only son, Lu Ten, died in battle. This led Ozai to request that his father, Fire Lord Azulon, revoke Iroh's birthright as crown prince, allowing Ozai to inherit the throne instead of Iroh. Ozai explained that without an heir, the line of Fire Lords would end with Iroh, while Ozai still had two children. Azulon became furious, refusing to betray his firstborn son's right to the throne. As punishment, Azulon ordered Ozai to kill his own son, Zuko, so he would understand Iroh's pain.
Princess Ursa found this out through her daughter Azula, and pleaded with Ozai to spare Zuko's life. Knowing that Ozai would do anything, even kill his own son to become Fire Lord, she offered him a plan that would ensure his rise to the throne and spare the life of Zuko. The princess concocted an untraceable color- and odorless poison and turned it over to Ozai to do "whatever [he wished]". Ozai agreed to this on the condition that his wife would leave the palace, believing he too might otherwise be poisoned one day. Ursa tried to take Zuko and Azula with her, but Ozai assured her they would be safe in his hands as long as she kept her word, and threatened to hunt them all down should she try taking their children. Shortly thereafter, Ursa made the poison and turned it over to him, and he sneaked into Azulon's room that same night to end his father's life with it.
The next morning, Azulon was found dead in his bed. His cremation took place a little later, and while Ozai and his two children were present, the Fire Sages announced Azulon's alleged dying wish; that Ozai would be his successor. The High Sage subsequently crowned the former prince Fire Lord, whereupon those present hailed him as their new ruler and bowed to him.
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