It's highly improbable in the context of the Harry Potter world. Hand - fasting is a positive and somewhat romantic or binding concept. Voldemort is not capable of love or normal human relationships. Daphne, on the other hand, has her own place within the Hogwarts student body. For her to be involved with Voldemort in such a way would require a complete rewrite of her character's values and motives, as well as a very strange and dark turn for the fanfiction plot.
No. Voldemort is pure evil and his goals are far from any form of normal relationship like a hand - fasting. Daphne is a minor character in the Harry Potter universe, usually portrayed as a typical Slytherin student. Voldemort's nature is so consumed by his thirst for power and destruction that it's hard to imagine him participating in such a ceremony. Fanfiction might try to explore this, but it goes against the core essence of what Voldemort represents in the original series.
In Harry Potter fanfiction, while it's an extremely unusual concept, it could potentially be possible in the realm of creative writing. However, Daphne is a Slytherin student and Voldemort is the ultimate dark lord. Their alliance in a hand - fasting (a form of betrothal or union) would be a very dark and unorthodox take. It would likely involve a lot of creative liberties with the existing lore, as Voldemort is mainly focused on domination and immortality, not on traditional relationships.
One way could be through dark magic coercion. Maybe Voldemort uses some powerful dark spell to force Daphne into this hand - fasting. Another possibility is that Daphne's family has some sort of dark secret or debt to Voldemort, and she is sacrificed in a way through this hand - fasting. But this would be a very dark and disturbing plotline in fanfiction.
There are not many fanfictions that would combine Daphne, Voldemort and Harry in the context of handfasting in a traditional sense. Handfasting is often associated with a form of magical binding or marriage - like ceremony in the wizarding world in fanfiction. However, if it exists, it would likely be a very dark and complex story. Maybe Daphne could be caught in the middle of the eternal battle between Harry and Voldemort, and the handfasting could be some sort of dark ritual Voldemort forces upon them. But this is all speculation as it's not a common or typical fanfiction trope.
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.
Themes could include the blurred lines between good and evil. If Harry is a fan of Voldemort, it challenges the traditional view of their roles. Another theme might be the exploration of dark magic. Since Voldemort is associated with it, Harry's fascination could lead to a deeper look into the nature of dark magic.
One possible plot could be Harry trying to understand Voldemort's past and motives through the letters. Maybe he writes to ask about Voldemort's childhood, what led him to become so evil. Another could be Harry attempting to persuade Voldemort to change his ways, using the letters as a form of communication to reach the dark wizard's humanity. Or perhaps Harry writes to taunt Voldemort, using the fanfiction as a way to get under his skin and make him angry in a psychological battle.
One possible plotline could be Harry being held captive by Voldemort in a way that's like a pet. Voldemort might use Harry to show his power over the Boy Who Lived. Another could be that Harry has some sort of magic that makes Voldemort keep him as a pet instead of killing him immediately.
One possible plot is that Harry discovers a hidden prophecy that says only by helping Voldemort can he truly save the wizarding world in the long run. So, he reluctantly starts to assist Voldemort in his conquests, all while secretly planning to turn the tables.
One consequence could be that the wizarding world falls back into chaos and fear. Voldemort would likely start his reign of terror again, targeting those who opposed him before. He might also try to seek revenge on Harry for initially defeating him.
The wizarding world would likely be in a state of shock and then celebration. People would start to see Azkaban in a new light, perhaps no longer as just a place of horror but also as the site where the great evil was finally vanquished.
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.