Even after thousands of years of human technological development and growth, surviving nature and various civilization collapses, humans weren't prepared for the Mana Advent. One day a godlike voice announced to the world of an update to the Universe itself, warning of various catastrophes. Over the course of the day, the world was filled with magic, triggering drastic changes to the world.
Civilizations were torn asunder, scattered to all corners of the globe, and the people inhabiting Earth were forced to survive and rebuild as the world's geography was thrown into disarray. However, the magic which has imbued itself within the world, also empowered various creatures which inhabit the world. Monstrous beings were born, and in no time at all the human population dropped. With most of their weapons and tools out of reach, and their livelihoods destroyed, the survivors must endure the hardships of the new world, and the magic it brings.
This setting contains a number of possible stories to be told, either taking place in the distant future, or following right after the Mana Advent. At the moment, I have one such story.
As magic flows into the world, a handful of teenagers end up the last unlucky few to be stuck at school. Without any parental supervision, the seven must band together to survive as monsters begin to gather around the school, and the buildings themselves seem to come alive.