Well, it could be that Voldemort has a strange fascination with wolves and decides to turn Harry into one as part of his plan to create a super - being under his command. Harry, in his wolf form, retains his own consciousness and memories. He has to endure the instincts of the wolf while also plotting against Voldemort. He might use the wolf's enhanced senses to gather information about Voldemort's lair and his plans, all while pretending to be under Voldemort's control.
One possible plot could be that Harry is magically transformed into a panther by Voldemort as a form of punishment or control. Harry, in his panther form, has to find a way to break the spell while also trying to gather information about Voldemort's plans. He might use his new form's abilities, like stealth and speed, to his advantage.
One possible plot could be that the contract is a magical binding that forces Harry to work with Voldemort in some way. Maybe it's a deal to save the lives of his friends. So, Harry has to reluctantly follow Voldemort's orders while secretly looking for a way to break the contract.
One possible plot could be that Harry, being Voldemort's little brother, has been hidden away from the wizarding world by some powerful magic. As he grows up, he starts to discover his true identity and his connection to Voldemort. This discovery throws the wizarding world into chaos as they don't know whether to trust Harry or see him as a threat.
In such a fanfiction, Voldemort would view Harry as his possession. He'd order Harry around, thinking he can use Harry's wolf - like abilities for his own dark deeds. However, Harry would not go down without a fight. He'd constantly be trying to undermine Voldemort from within. Their relationship would be a very strange and twisted one. Harry would have to pretend to be loyal while actually hating Voldemort with every fiber of his being. And Voldemort, being so arrogant, might not realize how much of a threat Harry still is even in his wolf form until it's too late.
One possible plot could be that Harry discovers a prophecy that ties his and Voldemort's fates together in a more intimate way than just being enemies. This leads him to seek out Voldemort to understand it better. As they interact, they find themselves attracted to each other. Voldemort, who has always been alone in his pursuit of power, starts to question his motives, and Harry tries to convince him to turn away from the dark path. The plot would be filled with internal and external conflicts as they navigate their relationship and the consequences it has on the wizarding world.
Perhaps the plot could start with Voldemort finding a way to capture Harry and turn him into a cat for his own amusement. Harry, as a cat, has to navigate the dark and dangerous world of Voldemort's lair. He could make unexpected alliances with some of Voldemort's minions who might have a change of heart. Meanwhile, his friends are desperately searching for him, not realizing he's been turned into a cat. As the story progresses, Harry could find a way to reverse the spell and use his new - found knowledge of Voldemort's secrets to finally defeat him.
One possible plot could be that they are forced to join Voldemort under some sort of dark magic compulsion. Maybe Voldemort has a new powerful spell that he uses on them to make them his servants. Then they have to struggle with their own moral compasses while doing his bidding, and secretly try to find a way to break free and defeat him from the inside.
One possibility could be that Voldemort rescues Harry Potter for some self - serving reason. Maybe he realizes that Harry holds the key to a greater power that he wants to obtain. So, instead of killing Harry, he saves him from another threat, like a powerful dark wizard who is trying to overthrow Voldemort and sees Harry as a useful tool as well. In this fanfiction, Harry would be in a very strange situation, having to trust Voldemort at least temporarily.
One possibility could be that Harry was somehow turned to the dark side early on. Maybe he felt betrayed by Dumbledore or the wizarding world. As Voldemort's right - hand man, he would use his knowledge of Hogwarts and the Order of the Phoenix to undermine their efforts. He might lead attacks on the school and help Voldemort in his search for the remaining Horcruxes.
In a fanfiction with this premise, the plot might involve Harry being raised by the Dursleys as normal, but then strange things start to happen that make him question his origins. Maybe he has some unexpected magical abilities that are a result of his unique parentage. He could then start to search for the truth, which leads him to uncover secrets about Severus and Voldemort's past relationship and how he came to be. Along the way, he would have to deal with the consequences of this new knowledge, both in terms of his own self - image and how the wizarding world views him.