Adolf Hitler is the Muggle slave of Grindelwald, kept under the Imperius Curse to do Grindelwald's bidding in the Muggle world. Right before he battled Dumbledore at the end of World War 2, Grindelwald stashed Hitler in a stasis chamber with orders to lead the German army in an attempt to slaughter the wizarding world if things went wrong.

However, in the battle with Dumbledore, the chamber was damaged, leading to Hitler not being released from stasis until shortly after Dumbledore's death.

Though the Imperio curse has mostly faded, its last remnants, plus temporary memory loss, drive Hitler to live like a wild animal in the forest around Godric's Hollow, near where his master once lived.

Harry and his friends stumble upon Hitler in their quest for the Hallows. While Hermione nurses him back to health, they must dodge Voldemort, the Ministry, and Isreali commando wizards as they fight to help Hitler recover his memory, reclaim his free will, and just possibly... recover a secret stash of German superweapons that will stop the Dark Lord for good!