Sozin

Sozin, great-grandfather of Zuko and Azula, grandfather of Ozai and Iroh, father of Azulon, was the Fire Lord who started the War with the other nations. During his lifetime, he secretly schemed to take over the entire world, but chose not to launch his attack until Avatar Roku was dead. Twelve years after Roku had died, Sozin used the power of a comet to enhance the power of his Firebender army and launched a genocide on the Air Nomads in an attempt to kill Aang, the new Avatar, and halt the Avatar cycle. However, Sozin failed, as Aang had run away from the Air Nomads some time before. Little is known of Sozin's role in the war afterwards.