Hermione was a, stubborn person. If she wanted to do something the best you can do is slow her down, or help her do it safely. She also had an unquenchable thirst for knowledge. So when her parents took her to a dentist's' convention in the Americas, she went to the nearest library, and practically lived there for the next few days.

That was how she learned of what would be the end of that universe. The Bermuda Triangle. Or more accurately, what is in the triangle. Hint: he is old, so very old.

The war was not going well, Ron has left. Only to return less than a day later to tell them that the rest of the weasleys were killed. They were hunted since the wedding, and now, he is the last. But even more, the impure blood was being executed by the ministary, and then made into inferi shock troopers. Using them to overpower protections with kinetic energy. Of course the public only knew that they were executed and sent to mass graves, not the reanimation or use of reanimated bodies.

But thats not all, The magical USA is in another civil war, Australia hates Britain, and Russia is locked tight. The rest of the major powers are in Europe, which is were Mean old voldie is. So no outside help.

The last of the order is hiding or dead. They. Had. Lost. Harry was broken when he heard how Remus 'escaped' his cage and bit into several ministry workers. He was executed shortly after.

Hermione was no better when she heard how the Australians returned her parents' memories, and forced them back to Britain at least all they took was an AK. Well that and the knowledge that Hermione left for them out in the middle of nowhere.

All three of them were broken. They slowly rebuilt themselves, but for one goal: Revenge. So when Hermione found a temple the the Bermuda Triangle to the Old Ones. She didn't hesitate to

use anything she found. And find she did. Voldi died the next day. His followers the day after. They were sacrifices to awaken the thing of Ry'lith, and he did awake. He consumed the world. Next: the stars. But he is a slow eater, so he may or may not still be eating humans.

Hermione had awakened Cthulhu. Or, more accurately, opened a portal and put up a marker basicly saying "plenty of food here".

That is how Hermione destroyed the Harry Potter universe