If Harry is the Master of Death in the fanfiction, the final battle with Voldemort would be a showdown on a whole new level. Harry would have a deeper understanding of the cycle of life and death. This could mean that he can manipulate the magic related to death in ways that Voldemort can't even comprehend. He could potentially disrupt Voldemort's connection to his Horcruxes, as he has a greater mastery over the essence of death that binds the Horcruxes. Moreover, Harry's power as the Master of Death could give him the ability to resurrect allies who have fallen in the battle against Voldemort, changing the tide of the fight completely.
In a Harry Potter final battle fanfiction, Harry might use some new and unexpected spells. For example, he could combine the Patronus Charm with a powerful offensive spell to repel the Death Eaters in a more spectacular way. Maybe he discovers a hidden power within himself that gives him an edge in the fight.
In this fanfiction, it could be that Voldemort has a premonition. He sees that if Harry Potter dies, it will lead to a series of disasters that will also destroy his own plans. So, in order to preserve the balance or to keep his long - term goals intact, he saves Harry. For example, if Harry's death causes an uncontrollable chaos in the wizarding world, Voldemort's rule over the dark forces might be disrupted. So he steps in to save Harry.
The wizarding world might be in shock. People who have always seen Voldemort as the ultimate evil would be confused. They would have to re - evaluate their views on Voldemort. Harry, on the other hand, would be in a very strange situation. He might be distrusted by some of his friends who think he has made a deal with Voldemort.
The relationship would become extremely complex. Harry would be confused and conflicted. He's been taught all his life that Voldemort is pure evil, but now he's been saved by him. Voldemort, on the other hand, might start to see Harry in a different light, perhaps as a tool or as someone with whom he can form an unusual bond. It would be a relationship full of mistrust on Harry's part and manipulative intentions on Voldemort's.
The relationship would become extremely complex. Harry would no longer be able to simply see Voldemort as pure evil. He would be in a state of confusion and moral dilemma. Voldemort, on the other hand, might start to view Harry as a more interesting pawn rather than just an enemy to be killed. They could develop a strange sort of respect for each other, at least on a surface level.
Perhaps in this fanfic, there has been some major plot twist that makes Harry believe that bowing to Voldemort is the only way to save his friends or the wizarding world. For example, Voldemort may have threatened to kill all of Harry's loved ones if he doesn't submit, and Harry is sacrificing his pride for the greater good.
In many fanfictions, after the final battle, Harry might focus on rebuilding Hogwarts. He could use his influence to make it a more inclusive and modern place of magic learning, with new courses perhaps on Muggle technology and its integration with magic.
Obviously, Harry Potter is one of the most important. His role as the Chosen One is central to the final battle. Hermione Granger also plays a crucial part with her intelligence and knowledge of magic. Ron Weasley too, as he provides support to Harry.
The house - elves could be unsung heroes. In fanfiction, they might use their magic in ways that the wizards and witches don't expect. Since they are so good at household magic, they could turn that to their advantage in the battle. For instance, they could make the Hogwarts castle even more of a maze for the Death Eaters to navigate, or they could use their skills to quickly repair any damage done to the castle's defenses.