Fire lord Ozai is the main villain of the whole series in Avatar: The Last Airbender. While his grandfather started the global war, he was the newest fire lord to keep it going.
We learned that he exiled Prince Zuko and gave him his infamous scar when his son refused to fight.
He didn’t have such a great relationship with his father either. We see the former fire lord unimpressed by his son and enraged when he suggests that Iroh should be disinherited because of the death of his own son.
So it’s strongly suggested that he killed his own father to take the throne
At the end of the series, he tries to take over the world as the Phoenix King, while giving the title of fire lord to his daughter Azula.
His battle with Aang was the only time we got to see him fight for the most part. His fire and lightning were crazy strong.
After being defeated, Aang takes away his fire bending.
I was glad to see him thrown in prison, powerless. His son was now Fire Lord Zuko and the two have a very interesting chat before the series ends.