God did not cause the apocalypse, nor was it manmade. Nuclear weapons didn't wipe out humanity, nor did the Earth heat up to unbearable temperatures. Continents didn't fall into the oceans, and volcanoes didn't block out the Sun.
In the end, it was mushrooms.
Nobody expected the third kingdom to be humanity's greatest enemy; we had always assumed they were just decomposers. Heck, we thought they were plants for most of our history. How could we expect them to be the smartest sentient life on Earth? That was our first mistake, thinking they were stupid. Individually, sure, they're useless, but each fungus is part of a whole, connected along roots that spread all over continents. They were the largest information gathering system the world had ever seen, and they wanted us dead.
The second mistake we made was thinking they were alone. After all, mushrooms didn't move. How could they harm us? We forgot all about cordyceps, the zombie mushroom. Capable of attaching to a creature and controlling its body, these tiny little parasites would spell doom for humanity. For years we knew they could control insects, but soon they evolved and started to control everything: raccoons, monkeys, dolphins. The only creatures immune seemed to be turtles and tortoises.
It wasn't long before even we started to succumb to cordyceps. They started to evolve. The infection is never pretty. Your skin becomes pallor, your eyes fall out, and the worse part is the mushroom caps that start to grow on your body, in sickening shades of all colors, spreading more of the parasitic fungi. At least you're not able to notice it. You're already under the influence of their hive mind that controls and conquers the living. Eventually the mushrooms didn't even need hosts to spread. They evolved to just move on their own. They were slow though, easily fought, but they could still catch you unaware, and that was that.
Only islands were ever really safe. Places like Hawaii, New Zealand, and Japan. I was stationed in Japan, a part of the Italian Embassy, which was a shame, as I never liked shiitake mushrooms, even before they started walking around. World leaders decided to put aside their differences and do what was best for humanity. There's a first. They assembled the able-bodied and sent them out to save and scavenge that which remained of humanity's legacy. Some were even sent to establish colonies, in hopes of once again ruling the world. It was hopeless though. Colonies never lasted, people were never saved. We were all that was left.
Until we found her. We were on an expedition in England, exploring the ruins of Stratford-upon-Avon. There was a sizeable colony there that managed to survive for a rather long time, but one day we lost contact. I was sent on a recon team to find out what happened, and if able, to rescue any survivors. It was usually a hopeless quest, just a reminder of what will happen to us if we too succumb to the mushrooms. But this time, hope did appear. For when we arrived, the mushroom people paid us no notice, as they were enthralled by something more tantalizing. They were chasing a girl, no older than five, who ran ahead them, crying as she fled. Eventually she noticed us and ran towards us, the horde of mushroom people following her. I ran forward to grab her, my brothers in arms raising there weapons to hold off the horde in front of us, but when I was just about to get a hold of her, she poked me. "You're it," she giggled, before running off, the mushrooms in pursuit. We stood in stunned silence, watching as she and the mushroom people ran about frantically, unaware that we weren't giving chase. She continued to cry out, which brought about a sickening realization. Her cries weren't of terror; they were of joy. She was laughing as she played amongst the dead. The mushroom people danced to her voice, bending to her childish will.
The mushrooms had evolved, but we had adapted.
My team brought her back to Japan, where we thought it was safe, but the people on the inside were as dangerous as the creatures out. The leaders of the world wanted to use her, dissect her and find out how she survived when others died, learn how she controlled the harbingers of death so that they could rule the ashes of our world, but we wouldn't let them. Armageddon occurred in Japan, between those who would kill a little girl and those who would protect her. The battle was short compared to other wars in human history, because this time we had the ultimate weapon; the mushrooms themselves. For every one killed, ten would take its place. Eventually, the war ended the way the first war began. Cordyceps. Using the sewer systems, we blasted cordyceps out into the civil centers of our enemies. We could hear their screams as they turned, their cries of denial as they slowly lost their humanity, soon to be replaced with the hollow moans of mushroom people. Our enemies had become our mindless allies. We had won.
And so that which remained of humanity vowed to protect her. We decided to give her life fit for a living god. Her new home was based in the National Diet, a palace in its own right. We built walls to keep out the dangers of the world, those who would try and take our princess. We raised her, spoiled her, and taught her to use her powers for the good of her kingdom and humanity. Her caretakers would tell her stories native to Japan, in hopes that she would emulate their lessons. Benkei, the Tale of The Bamboo Cutter, and Urashima Taro: loyalty, duty, and caution. Her favorite story was Momotaro, the peach boy, and so she became our Momohime, our peach princess.
Our Princess Peach.