Avatar: The Last Airbender Season 3 Episode 6 : The Avatar and the Firelord
Aang and Zuko learn about Avatar Roku and Firelord Sozin's shared past which resulted in the War. Their childhood friendship, their falling out, and Sozin's betrayal of Roku to his death. Zuko discovers that Roku is his maternal great-grandfather, giving him significant insight into his internal struggle. Iroh encourages him to do the right thing by giving him an artifact that Avatar Roku wore in his lifetime.