While inside the Titan’s stomach, surrounded by the corpses of his fallen comrades, Eren’s intense rage and desire to "kill every last one of them" triggered his latent power. He unknowingly fulfilled the two requirements for Titan shifting:
There are pivotal moments in anime that redefine the genre, and then there is Eren Jaeger’s first transformation in Attack on Titan . Up until this point, Attack on Titan had been established as a brutal, hopeless survival horror. Humanity was cattle; the Titans were insurmountable gods. When Eren is swallowed by the Bearded Titan while saving Armin, the show seemingly adheres to the "anyone can die" trope it had just established with the death of Marco and the trainee squad. eren turns into a titan season 1
Eren’s first transformation is a masterclass in pacing and subversion. It saves the remaining cast from certain death, but it also dooms them to a new reality where the enemy might be human. It is the moment Attack on Titan cemented itself as a modern classic. While inside the Titan’s stomach, surrounded by the
Finally, it introduces the true antagonists. The season’s climax reveals that two other shifters have been living among the cadets: Reiner Braun and Bertholdt Hoover, the Armored and Colossal Titans. Eren’s transformation is the catalyst that forces them to reveal their mission: to capture him and “save the world.” The fight is no longer humanity vs. Titans, but one faction of shifters vs. another. Humanity was cattle; the Titans were insurmountable gods
His “death” serves a crucial narrative purpose. It strips away the expectation of a traditional hero’s journey, leaving Armin and Mikasa to grapple with grief amidst the chaos. However, moments later, a new, unusually coordinated Titan appears—a fifteen-meter-tall, muscular, jawless creature with long, black hair and piercing green eyes. This Rogue Titan immediately attacks other Titans, displaying a focused aggression and tactical awareness no mindless Titan has ever shown. The mystery is solved when the Rogue Titan punches a hole in a wall, revealing a steaming, unconscious Eren Yeager emerging from its nape. The shocking truth is laid bare: Eren is the Titan.