In a 'Harry Potter and Voldemort team up' fanfiction, the plot could involve a prophecy that states only the combined power of the two can save the wizarding world from an upcoming cataclysm. At first, they are reluctant, but as the signs of the impending doom become more evident, they put aside their differences. They use their combined magic to unlock ancient secrets and find magical artifacts that are needed to prevent the disaster. This new alliance also causes a major shift in the power dynamics and relationships within the wizarding world.
A group of rogue Death Eaters could be the main antagonists. They think that Voldemort has gone soft by teaming up with Harry. So they break away and form their own more extreme and violent group. They start attacking both the wizarding world and anyone associated with Harry and Voldemort's new alliance. Harry and Voldemort must then fight against their former allies who have become a greater threat than ever.
One possibility could be that Voldemort has a sudden change of heart. Maybe he realizes that Harry is the only one who can truly understand his loneliness and power. So instead of killing Harry, he decides to save him from a dangerous situation, like a powerful dark magic that even Voldemort fears could destroy the wizarding world if not controlled, and in the process, they form an unlikely alliance.
Maybe a plot could involve a powerful spell that binds their fates together and they have to learn to live with each other. Eventually, they might start to understand each other's motives and fall in love.
Perhaps Harry joins Voldemort out of a sense of hopelessness. After seeing so much destruction in the wizarding world, he might think that joining Voldemort is the only way to end the war quickly. However, he soon realizes that Voldemort's methods are too extreme and tries to find a way to turn things around from within the dark side.
It's a tough one. Maybe if Voldemort was somehow forced to change his ways by a powerful magic, Harry could team up with him. But it would be really hard for Harry not to feel like he was betraying his principles. After all, Voldemort has killed so many people.
Perhaps in this fanfiction, Harry discovers some dark secret about the Order of the Phoenix or Dumbledore that makes him lose faith in the side he has always been on. This, combined with Voldemort's manipulation, could lead him to side with the Dark Lord. For example, if he finds out that Dumbledore has been using him all along without his full knowledge, he might be so disillusioned that he turns to Voldemort as an alternative.
In this fanfiction scenario, it could be that Voldemort was misunderstood due to a long - standing feud between two powerful wizarding families. The real motives of Voldemort were to bring equality among all wizards, regardless of their blood status. Harry, who has always fought for justice, sees the potential in this 'good' Voldemort and decides to join him. They then start a journey to change the perception of the wizarding society towards Voldemort and his ideas.
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 possible plot could be that Harry discovers the truth about his heritage as he gets older. This causes a huge upheaval in the wizarding world. His friends might be initially shocked and some might even turn against him. Harry then has to struggle to prove that he's not like Voldemort while also trying to understand his new - found identity.
Well, it could be that in this fanfiction, Harry discovers some long - hidden truth about the wizarding world that makes him realize Voldemort's motives aren't entirely baseless. Maybe there's a prophecy misinterpretation that leads Harry to think they can work together to fix the wizarding world's problems.