the story begins from the events of Half Blood Prince, when dumbledore senses his last breath is near. And with him gone forever the world will face something terrible(not just the wizarding world but muggles too). and this danger is far more dangerous than Grindlewald or Voldemort. In short they are like just born kids to the danger, which he saved from the world to be known. but for now all he can do, is believe in his mates helping him for years to save the danger from ever releasing. Although the hardest time anyone has seen is During the rise of Voldemort and the second wizarding war.
As he knew there's no right time to take good steps with harry or anyone to tell them about the secret Danger waiting to be released, he accepted his Death knowing the fact that soon after his death, Voldemort's era will be over.
After recuiting severus to tell harry everything he needed to know to defeat Voldemort, Dumbledore wrapped up a secret gift for harry to recieve after he's gone and the war with Voldemort is over.
the secret he wanted to reveal is...