Well, it all happened when Eren got hold of that special serum. When he used it, it activated a genetic trait within him, allowing him to transform into a Titan. It was a crucial moment in the story that changed everything for him.
The Attack on Titan manga concluded with 139 chapters. It's been a long and intense journey for fans. Each chapter added depth and complexity to the story.
The Attack on Titan manga has a significant number of chapters. Up until a certain point, it was around 150. But new chapters could have been added since then.