In some fanfictions, there could be a magical bond between them that forces Voldemort to heal Harry. This bond could be something that was created by an ancient magic or a prophecy. It's a creative way for the author to explore the relationship between the two characters in a new and unexpected way. For example, the bond might be related to the fact that Harry is a Horcrux, and Voldemort needs to keep Harry alive to keep a part of his soul intact, but also wants to use Harry's power for his own ends. So, he heals Harry to ensure his survival while also trying to manipulate him.
Well, in some fanfictions, it might be justified by a complex backstory. Maybe Voldemort realizes that Harry has some power or knowledge that could be useful to him in a different way than just killing him. So, instead of harming Harry, he decides to heal him to gain an advantage.
One common theme is redemption. It shows Voldemort having a chance at redemption by saving Harry. Another could be the exploration of power dynamics. With Voldemort healing Harry, it flips the traditional power relationship on its head.
In fanfiction, the author often has the creative freedom to play with the rules. So, it could be that there is a new form of magic introduced that is unique to the Azkaban setting in this story. This new magic might have affected Harry in such a way that he becomes pregnant. It could also be related to the strange and often malevolent energy that surrounds Azkaban, which has some sort of mutagenic effect on him.
In such a fanfiction, perhaps Harry uses his magic skills learned at Hogwarts. He might first find the injured unicorn in the Forbidden Forest. Then, using a spell like 'Episkey' or some advanced healing magic he's discovered. The unicorn could be injured by dark creatures or a trap set by poachers.
Well, perhaps Harry gets lost in a magical portal and ends up in Middle - earth near Rohan. He finds Theoden ill or injured. Harry uses his magic, like the spell 'Ferula' to bind wounds or 'Brackium Emendo' to mend broken bones. Then he has to deal with the Rohirrim's suspicion of his strange magic.
One way it could be justified is through a dark magic plot. Maybe Voldemort uses some powerful and ancient magic to bind Hermione to him against her will at first. But as time goes on, she starts to understand his past traumas which led him to become so evil, and in a twisted way, tries to reform him. However, this goes completely against the original good - evil divide in the Harry Potter series.
The fanfiction might start with Harry being haunted by the fate of the Longbottoms. He realizes that he has a moral obligation to try and help them. As he begins his journey to heal them, he uncovers secrets about the dark magic that was used against the Longbottoms. These secrets could be related to a particular Death Eater or a forgotten branch of dark magic. Along the way, he has to also deal with his own insecurities and doubts, but with the support of his friends, he perseveres until he finally manages to heal the Longbottoms.
Well, one plot point could be Harry discovers a unique spell to heal the unicorn. Maybe he has to face some obstacles like dark creatures trying to stop him while he's on his way to heal the unicorn. Another could be that the unicorn has a special connection to Harry, like it can communicate with him telepathically and gives him clues on how to fully heal it.
One common plot device is the revelation of Snape's hidden motives. As he heals Harry, we might discover that his actions are not just out of duty but also out of a long - held, unspoken love or loyalty. Another is the use of rare and dangerous ingredients in the potions he makes for Harry's healing.
In a 'harry heals a unicorn fanfic', perhaps Harry uses his magic skills and knowledge. He might find an injured unicorn in the Forbidden Forest. He could use spells like 'Episkey' or some advanced healing magic he has learned. Then he has to deal with the unicorn's trust issues as they are very shy and skittish creatures.
In a 'harry joins voldemort fanfiction', perhaps Harry discovers a dark secret about the wizarding world that makes him think Voldemort was right all along. It could be something about the Ministry of Magic being corrupt and Voldemort's way of ruling, although extreme, is the only solution. So he joins Voldemort to 'fix' the wizarding world.