Perhaps the story takes a different turn in terms of the main character's goals. In previous volumes, he might have been focused on one aspect of building his power, but in Vol 11, he could be branching out into new areas. So, the direction of the plot is different.
It could have a more complex plot. Maybe it builds on the events of the previous volumes and takes the story in a new direction.
It could be the new plot twists. Maybe in volume 5, a character who was previously an ally turns into an enemy or vice versa. This can completely change the direction of the story.
Again, not having read it, it's hard to say precisely. But usually, the story in Overlord progresses by building on the previous events. If in the previous volumes Ainz was establishing his base, in Vol 17 he might be moving towards more outward expansion or dealing with the consequences of his earlier actions.
In Saga of Tanya the Evil Vol 9, it might be special because it could show a more in - depth exploration of Tanya's character. As the story progresses, her experiences in this volume could change her in ways that were not seen in the previous volumes. Also, the world - building might be enhanced, with new aspects of the magical or political systems being revealed that add more depth to the story compared to earlier volumes.
It might have a more complex plot compared to the previous volumes. Maybe it builds on the events of the earlier ones and takes the story in a new direction. The characters could also be more developed, having learned from their experiences in previous volumes.
It might have a more complex plot. As the series progresses, the story often gets more intricate, and vol 8 could be no exception.
It's more complex. The plot in vol 6 builds on what has come before and gets more intricate. There are more layers to the games and the strategies involved.
One possible difference could be the new antagonists. Each volume often has different foes for the main character to face, and Volume 7 might introduce some unique villains.
In terms of plot, it might be more convoluted. The previous volumes have laid the foundation, and this one could build on that with more intricate storylines. Maybe there are more sub - plots that intertwine with the main plot, making it a more engaging read.
Compared to the previous volumes, 'A Certain Magical Index Vol 18 Light Novel' could introduce new concepts and ideas. The character relationships might have evolved further. For instance, some characters who were once enemies might start to form alliances or vice versa. The overall world - building could be more detailed, with new locations and magical phenomena being explored.