Geological / environmental event
The skies finally clear and the survivors emerge, forever changed.
For the first time in an age, the sun once again shines on Earth. What's revealed is a feral and brutal world. The hard men and women that survived were tough and vicious with some having turned to cannibalism. Humanity finds itself sparsely scattered across the world and isolated from each other. As the environment recovers, man begins to explore the Earth for the second time in history.