Avatar: The Last Airbender Season 2 Episode 20 : The Crossroads of Destiny
With Long Feng released, he presumably retakes leadership of the Dai Li. However, Azula explains that the Dai Li are not convinced on who to take orders from yet. She betrays Long Feng by revealing his history of conniving and cheating his way to power, forcing him to step down permanently as leader of the Dai Li. When the group attempts to stop Azula, Zuko, after some deliberation over the consequences of his actions, betrays his uncle's trust and chooses to attack the Avatar. Aang is fatally injured by Azula while in the Avatar State, but Iroh intervenes and gives his friends enough time to escape. With the Earth King overthrown, Ba Sing Se falls to the Fire Nation. Katara uses the spirit water to heal Aang and he survives.