If you are looking for the Eren transforms into a Titan, it happens in:
Eren, fueled by a blinding rage against the "animals" who destroyed his home, charges headlong into battle. He is not a hero yet; he is a reckless, grieving teenager. His squad is decimated. His comrade, Thomas Wagner, is snatched and killed in front of him. In a moment of distraction—a rookie mistake—Eren looks away from the nape of a 15-meter Titan.
Unlike Naruto’s Nine-Tails or Ichigo’s Hollow, Eren’s Titan power is not a cool power-up. It is a curse of conscience. The "when" of his transformation is the moment Attack on Titan stops being a story about killing giants and starts being a story about the cost of freedom. in which ep eren become titan
In the darkness, Eren hallucinates a memory—his father, Grisha Yeager, injecting him with a mysterious serum in the woods behind the chapel. He sees the key. He hears the words: “Their memories will show you how to use this power.”
Inside the Titan’s steaming stomach, Eren is not dead. He is suspended in a nightmare of heat, acid, and viscera. This is the feature’s crucial visual metaphor: If you are looking for the Eren transforms
Eren’s leg is bitten off. Blood sprays across the rooftop. Before he can even scream, the Titan lifts him into the air. Commander Keith Shadis’ earlier warning echoes prophetically: “What separates us from the Titans is our willpower. Lose that, and you’re nothing but a piece of meat.”
Eren Yeager transforms into a Titan for the first time in Episode 1 of the anime series, which is titled "Struggle: A Desperate Escape". This episode originally aired on April 7, 2013. His comrade, Thomas Wagner, is snatched and killed
It is ugly. It is clumsy. It is not heroic. It is the raw, physical manifestation of trauma.
In this episode, Eren and his friends Mikasa Ackerman and Armin Arlert are on a mission to rescue Eren's mother, Carla Yeager, from a Titan that has invaded their hometown, Shiganshina. However, during the battle, Eren is confronted by a massive Titan, and in a shocking turn of events, he transforms into a Titan himself, much to the surprise of Mikasa and Armin.
It is not a battle cry that saves him. It is rage refined into absolute purpose.