His fiery berserk mode battle against the Female Titan in Stohess.
In the popular anime and manga series "Attack on Titan," Eren Yeager's transformation into a Titan is a pivotal moment that significantly impacts the storyline. This transformation occurs in the first season, specifically in Episode 13, titled "Primal Desire: The Struggle for Trost, Part 9." in which episode eren turns into titan
Though not a physical transformation, this is when he first uses the "Coordinate" power. His fiery berserk mode battle against the Female
However, for those inquiring about Eren's controlled transformation or significant revelations related to his Titan form, it might be more about or later episodes where more details about his transformation are discussed. specifically in Episode 13