Challenge #1:

Summary: After Dumbledore's death, the wizarding World shatters. With no one left to oppose him Voldemort goes into the open.
The Ministry falls not through corruption, but incompetent organization, lack of manpower and lack of courage.

Grimauldplace Number 12, the Headquarter of the Order of the Phönix, has fallen. With no safe haven and no leader the light side keeps arguing among itself. Hogwarts is under Voldemorts control, Muggleborns are hunted down and killed.
And, worst of all, the Horcrux hunt is stagnating.

In an act of desperation the Golden Trio goes of to free the only Wizard alive who could still fight Voldemort: Grindelwald.

It goes well at first. Grindelwald agrees to help them, he shows regret for his deeds and even compassion towards Dumbledore.
But then his methods become more and more cruel. Harry has to ask himself if the price was worth the cost, what to do when "it's not right" won't convince your ally.
And what Grindelwald will do once the war is over.

Rules:
- No major resistance against Voldemort
- Anarchy
- Grindelwald is freed in an attemt to give Voldemort a big enemy

- No Muggle involvement
- Grindelwald really does fight against Voldemort, but still employs a "for the greater Good" policy"
- He is not convinced by Harry, or anyone

 

Isn't that like unleashing Hitler to fight Bin Laden?

 

Desperate times call for desperate measures. They unleash the second worst dark wizard in the history to deal with the worst one and hope that they both destroy each other. And don't destroy the world while they fight.

 

The spelling mistakes in this made me dubious but the idea has some potential. Maybe Harry's slowly taken in by Grindelwald, not realising that he hasn't really changed until he's gathered a new army and that he - Harry - has spilled innocent blood.

 

Well, this idea could use some better fleshing out. How would the trio know Grindelwald is still alive? When people think 'defeated' they usually think of killing the person. Of course, Dumbledore realizes that you don't have to kill to defeat someone, a sentiment most don't realize.

So, here's what you do. Harry realizes he needs something to even the field with Voldemort, and the hunt for Horcruxes is going stale. So, he decides on "Hallows, not Horcruxes", and does some research. Blah Blah, tracks down the elder wand through history, concludes that Dumbledore has it. Dumbledore dies, and Harry goes through his memoirs and stuff, and realizes that Grindlewald is still alive, prisoner in his own prison. Then, commence the Jolly Cooperation.