Fire Lord Azulon
Fire Lord Azulon was the son of Fire Lord Sozin, who had originally started the war. Azulon was married to a woman named Ila, was father to Iroh and Ozai, and was grandfather to Lu Ten, Zuko, and Azula. He ruled over the Fire Nation for 23 years, and is remembered as a "fearless leader in the battle of Gar Sai" and a "matchless conqueror of the Hu Sin provinces."
Not much else is known about him except that he was in good health and very serious-minded. He didn't like to have his time wasted, and got annoyed when others didn't get to their point. He also seemed to be more favorable of Iroh, and was once considered to be very powerful.
The day before his death, he held audience with Ozai, Ursa, Zuko, and Azula. Ozai tried to impress him with Azula's talents, but after Zuko's failed attempt to do the same, Azulon ordered them to leave so that Ozai could get to the reason for their meeting. Tactfully, Ozai pointed out that with Lu Ten's death, Iroh's bloodline had ended. He had abandoned the siege at Ba Sing Se, and it wasn't known when he would return home. "But I am here, father, and my children are alive."
Irate, Azulon told Ozai to say what he wanted, and Ozai asked for him to revoke Iroh's birthright and use him (Ozai) instead. Shocked and angry, Azulon replied, "You dare suggest I betray Iroh? My first born? Directly after the demise of his only beloved son? I think Iroh has suffered enough. But you, your punishment has scarcely begun!"
Zuko and Azula had been spying on the scene from behind a curtain, but at Azulon's fiery outrage, Zuko ran off while Azula stayed to watch. A while later she came to Zuko's room, taunting him that their dad was going to kill him. She went on to say that Grandfather said their dad's punishment should fit his crime. "You must know the pain of losing a first born son, by sacrificing your own!" Ursa came in soon after, and lead Azula away to have a talk.
Whether Azula was telling the truth or not is unknown, but the next day Ursa was no where to be found and Fire Lord Azulon had died. His funeral was elaborate; many people attended and the Great Sage from Avatar Rokuís temple spoke his eulogy. In the Fire Nation tradition, Fire Lord Azulon's sarcophagus was set aflame, and as was his "dying wish", Ozai was crowned Fire Lord.