Fire Lord Sozin was the instigator of the Fire Nation War and the Fire Lord before his son, Azulon. He was grandfather to Iroh and Ozai
As a young prince, Sozin was best friends with Avatar Roku. After Sozin took up the throne and Roku became a Fully Realized Avatar, their friendship became strained to the breaking point as Sozin desired the Fire Nation to encompass the world, a plan to which Roku was strongly opposed. Following a confrontation over this which ended in Sozin's defeat, neither spoke to the other until they were both very old. Sozin betrayed his old friend to his death and began waging war on the other nations.
Using the fire-enhancing power of a comet, Sozin attacked the Air Nomads in an attempt to eliminate the next Avatar, but failed. He died before the war ended in 10 ASC.