Ginny took out her diary, and looked fondly at the cover of it, the faded gold lettering saying 'the diary of Gellert Grindelwald.' Ginny had seen the book a few years ago in Borgin and Burkes and had had to buy it. After all, what could go wrong from owning the old diary of a powerful dark wizard.
As it turned out, the part of Grindelwald's soul trapped inside the horcrux hadn't tried to possess Ginny even once. It was quite friendly to be honest, and knew just how to cheer Ginny up after a he's day at work. Of course there was the occasional mention of muggle genocide or the superiority of wizards, but Ginny just ignored the old man and steered the conversation back to her. (Although she never once thought to ask how the diary was feeling, bit of a dick move to be honest).
Recently Ginny hadn't had too much reason to write to the diary, after Fred's return from the forest her life had been pretty happy. Today was an exception however, especially after she had been on reddit (a far darker place than the world under any dark wizard) and had seen how much muggles truly despised gingers. Oh and also wizards decided to stop being idiots and finally adopted muggle technology because who really wants to live in a world without Google.
This anger lead to her humouring Grindelwald's murderous ramblings today, and she imagined herself, raining terror down upon the muggles that had slighted her hair. As she listened to more of what Grindelwald had to say, she realised that he was making more and more sense. After much deliberation she decided to help restore Grindelwald to his full strength. After all, what was the worst that could happen.
The first thing that she did was break into the local newsagents and steal one of their time turners (because apparently they're just lying around everywhere). She then transported herself , with the diary back to 1997 and politely asked the dark lord of the Sith, uh, I mean, the dark wizard Voldemort for help. Of course he was only too happy to further the cause of killing muggles and at no point questioned why Ginny was suddenly genocidal. Voldemort used the simple charm avada kedavra. It was strange, he thought, that this charm wasn't used more often, it could potentially save people a lot of Fred-related problems in the future.
With this, Grindelwald was returned to full strength. Of course Voldemort gave the elder wand to Grindelwald. Gellert had owned it first afterall and it was only fair. With this done they thanked the kind wizard and used the time turners to travel back to the future (queue theme music).
Once returned to the present day Ginny first took Grindelwald to Fred's clearing (he had been refusing to sleep anywhere other than a forest since he had returned from living with the centaurs. She immediately proceeded to kill him in his sleep because Ginny was still bitter that he hadn't taken her to live with him in the forest so that she wouldn't have had to have dealt with all that bullshit about a vaguely not great child. (As was now the politically correct term since cursed was seen as derogatory).
Following this, she and Grindelwald travelled to London and started Avada-kedavra-ing left right and centre but after a couple of hours the muggles started to work out that they could just go down a different street and the rate of killing started to slow down a little. It was at this point that Grindelwald came up with a mildly genius and completely foolproof plan.
They would steal the power of the greatest wizard of all time: Merlin. They travelled back to the totally real times of Camelot and the round table where they easily found Merlin as the population of England at the time was only about 43.
There was then an epic battle between the greatest wizard of all time, who was said to be unbeatable and the most powerful wand of all time, which was also unbeatable. Ginny helped too probably but at this point, who really gives a shit about her.
The battle carried on for an age, no wizard able to get the better of the other. It was a truly awe-inspiring battle, the likes of which had never been seen before and would never be seen again. Unfortunately, this has been dragging on for forever and I really can't be fucked to describe it. Suddenly though, during the 52nd year of combat, Merlin died of old age. Grindelwald staggered out of the smoke and rubble that went on for miles, a small indicator of the epicness that you just missed.
He was clenching in his hand the source of Merlin's near limitless power; his beard. Small strips of skin still hanging on to it where the beard had been hastily hacked off by Grindelwald. With his newly discovered power he travelled back to the future again and proceeded to start killing muggles again. Eventually, he realised that Merlin's beard had nothing to do with his power and Grindelwald had just wasted most of his life for some hair extensions.
At this point Grindelwald did the smart thing and took to Google where he searched for the best ways to take out the human race. He saw that North Korea had some potential but after a little more research he decided that the best way would just be to elect some reality tv star to be president and let him blow him the world himself. Eventually though, Grindelwald realised that this plan was far too topical so he just went back to the Cold War, slightly nudged events during the Cuban missile crisis and the world promptly proceeded to blow itself up. At this point Grindelwald realised that he had fucked up and that wizards may not be able to survive a small tactical nuke. This story was far too long already though, so he just sat down, and waited to burn.