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.
Well, sometimes he uses his titan's speed and agility to dodge the Colossal Titan's attacks and then strike at its nape. It's a classic way to take down a titan in the Attack on Titan universe.
One possible fanfic plot could be that Eren has been planning this for a long time. He might gather intelligence about the Colossal Titan's weaknesses. Then, in a fierce battle, he manages to get close enough to deliver a fatal blow. This could be a very emotional moment for him as well, considering the history between them.
One popular fanfiction idea could be a story where Eren has to team up with the Female Titan against a new, more powerful enemy in the world of Attack on Titan. Maybe they start off as reluctant allies, but as they face challenges together, they begin to understand each other better and develop a unique bond.
One common theme is the struggle for identity. Eren being a titan makes him question who he really is. Is he still the same Eren or has the titan side changed him completely?
Love and friendship are also prevalent themes. Despite their differences and the initial hostilities between them, they might develop a deep friendship or even romantic feelings. This can add a whole new layer of complexity to the relationships in the Attack on Titan world. For instance, they could be the key to uniting different factions within the story that were previously at odds because of their relationship.
The love story between them develops gradually. At first, Mikasa's love is more of a protective instinct towards Eren. As they face more and more challenges together, like battles against Titans and political unrest, their bond deepens. Eren starts to understand Mikasa's feelings more, and they share some tender moments. However, Eren's changing goals and the overall complex situation of the world they live in also put strains on their relationship. Their love story is a complex journey of growth, understanding, and sometimes, heartbreak.