Some fans are heartbroken. Eren is a very popular character, and his death can be a huge emotional blow. They might write comments expressing their sadness and disappointment.
In many fanfics where Eren dies in Attack on Titan, it can be a very emotional and thought - provoking experience. Some fanfics might focus on how his friends deal with his death. For example, Mikasa could be in deep mourning, and Armin might struggle with the loss while also trying to uphold Eren's ideals in a different way. It could also explore the political and social ramifications in the world of Attack on Titan after Eren's death, like how the different factions react to the end of the great threat he represented.
In this fanfiction, his grandparents might be described as having a deep connection to the magical land they live in. They could be the keepers of ancient family secrets. Their physical features might be aged but still show signs of the great beauty and strength they had in their youth. And they may be very loving towards Harry, making up for the lost time they could have had with him.
In some fanfics, Eren's death could be a heroic sacrifice. For example, he might give his life to finally defeat all the Titans and save his friends and humanity. He could use his Titan powers in a last - ditch effort that consumes him completely.
Perhaps his grandparents were great adventurers in their time. They could have traveled to far - off magical lands and encountered strange and wonderful things. Their experiences could be passed on to Harry, inspiring him to explore more of the wizarding world and its secrets. They might also have been involved in some great battles in the past, and their war stories could give Harry valuable lessons for his own fight against Voldemort.
They could be very traditional and strict pure - blood wizards. Maybe they were part of some ancient magical order and had high expectations for the Potter family. They might be disappointed in how James and Lily Potter deviated from some of the old ways, but be very proud of Harry when they get to know him.
They can document everything. Keep records of their relationship with the grandchild, like photos, videos, and any communication. If there's a court case, this evidence can be very helpful. For example, if the custodial parent claims the grandparents are a bad influence, but there are pictures of them happily baking together, it shows otherwise.
Often, in these fanfictions, Eren develops more empathy. He starts to really understand Mikasa's feelings. For example, he might realize how much his actions impact her and become more cautious. He becomes more of a protector instead of just a fighter.
He might be more old - fashioned in his ways. His grandparents' values would likely have a big influence on him, so he could be more reserved and traditional compared to the Harry we know from the original series.