In the Crap Times, magic flew wild and loose around the earth. It changed and killed, created and destroyed. Cities could be gone in an instant, or un-gone in the next. Nuclear fallout mingled with arcane potential, and nothing was the same.
The blasts were worst at the cities. Some of them stopped existing, some of them kept existing, but only in different places, and some of them stopped ever having existed. And the people who lived in them were even worse off.
Some people who didn't live in the cities survived. Sometimes they lived because they hid, deep in the mountains or the forests. Sometimes they lived because they were lucky. Sometimes they lived because what the magic changed them into helped them to live. But after many, many years of the Crap Times, it seems the irradiated magic has settled, and the survivors of what was once the human race are starting to leave their enclaves, venturing forth into the Now What?