Eren's death in the manga was a powerful moment. It was brought about by a combination of his own decisions, the opposition he faced, and the overall plot developments. It left a lasting impression on readers.
Eren managed to beat Annie through a combination of his determination, newfound powers, and strategic thinking. He exploited her weaknesses and caught her off guard.
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.
You can start by looking on popular fanfiction websites such as Archive of Our Own. There, you can simply search for 'grisha' and a whole list of fanfictions will come up. You can then sort them by popularity, rating or length to find ones that might interest you.
Eventually, Eren might start to investigate the reasons behind Mikasa's betrayal. If he discovers that there are more complex factors at play, like she was forced or there was a misunderstanding, he could feel a mix of guilt for his initial reaction and relief that their relationship wasn't as broken as he thought. However, if he finds out that she truly betrayed him out of her own will, he might become even more determined to carry out his plans, whatever they are, without her.