Many years ago, the world was suddenly overcome with massive amounts of corrupted magic, killing off most living beings on the planet, and tainting the survivors so much, that they instantly mutated into all sorts of monsters. What was once the Earth, with its blue skies, green grass, and complete lack of magic, became a wasteland filled with all sorts of dangerous creatures. That disastrous event was later called the "Great Corruption".
Flora, fauna, even people—nothing was free from the corruption. Monsters started roaming the world and attacking people, demons suddenly appeared from another world for reasons still unknown, and the sky was permanently tainted with corrupted magic, giving it an ominous dark gray color.
Humanity is no more, and in their stead are a variety of mutants, monsters, and other paranormal creatures. But if there is one thing that has always been a constant for people, human or not, it is their adaptability.
And so, life goes on.
("Corrupted World" is its own universe, separate from my other ones. Unlike the others, which are story-oriented, this one is essentially a sandbox. It has stories, but it isn't one itself.)