Reiner did die in the manga. His death was a result of a series of events and conflicts that had been building up throughout the storyline. It had a major impact on the plot and the other characters.
Once upon a time, in an old, abandoned mansion on the outskirts of a small town, there lived a dark presence. People said that late at night, you could see a figure in the attic window. A young girl, curious about the rumors, decided to explore the mansion one night. As she entered the dusty hallways, she heard strange whispers. The further she went, the colder it got. Suddenly, a cold hand grabbed her ankle. She screamed but no one could hear her. She was never seen again.
There was a dark forest where no one dared to go at night. Legend had it that a witch lived deep within. One day, a group of hikers got lost and ended up in that forest. As night fell, they started to hear strange noises. Branches were snapping as if something large was moving. Then, they saw a pair of glowing eyes in the bushes. One of the hikers shone a flashlight, and for a brief moment, they saw a hideous face with long fangs and matted hair. They ran blindly through the forest, not knowing if they would ever find their way out.
If Lord Reiner is from a particular book or game, more context is needed. For example, if it's a character from a fantasy novel, we would need to know the plot of the novel, the world it is set in, and his relationships with other characters to tell his full story.
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.