Harry might use his wits and the environment around him while in captivity. For example, if he's held in a place with magical artifacts, he could use them to his advantage. He could break the chains that bind him with a spell - enhanced object and then make his way to Voldemort and defeat him.
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.
Because the Harry Potter series has a huge and passionate fan base. The world J.K. Rowling created is so rich and detailed that fans want to explore different aspects of it through fanfiction. They might want to see different character pairings, new adventures for the characters, or explore what could have happened if certain events in the original series had gone differently.
Perhaps he wants to start a new school or a training center for young wizards and witches, and a mansion would be a perfect place for that. It could provide ample room for classrooms, dormitories, and training grounds.
It could be part of a spell or a magical prank. Some fanfictions like to play with the idea of magic being used in creative and humorous ways. A spell might have gone wrong and made Harry want to wear a dress, or it could be a prank by Fred and George where they cast a spell on him to do so.
It could start with a lot of animosity on Harry's part, but as he is trained, he might start to see Voldemort in a different light. Maybe he begins to understand Voldemort's motives, though not necessarily agreeing with them.
He might want a place of his own, away from the chaos of Hogwarts and the wizarding world's constant threats. A building could be his private sanctuary where he can relax and practice magic without being bothered.
One possibility is that Harry has discovered some long - lost connection between them that he thinks can be exploited. Maybe he's found out that Voldemort has some knowledge or ability that is crucial to solving a problem. For instance, if there's a prophecy that only Voldemort can decipher, Harry might approach him for help, despite their long - standing enmity.
Harry could become more powerful in a way that scares his friends. Voldemort's training might focus on the Dark Arts, which are powerful but dangerous. This could change Harry's magic style. He may become more solitary as his new skills and knowledge set him apart from his Hogwarts friends who follow a more traditional path of magic.