Homebrew world with some mechanics lifted from other places:
Something came crashing down to the earth (Spoiler: it was the corpse of Ymir, the frost giant god) causing an impact winter (a scary thing, like nuclear winter). People are just trying to survive.
You have remnants of the halflings who made a last stand to save the other 'good races'. You have orc hordes whose savage attacks made the wood elves turn to Yeenoghu for powers to fight back. You have an alien infection causing a fast-zombie plague. The game started when the refugees arrived at a secluded glade that a druid had channeled his magic into a great tree which keeps most of the temperature issues at bay... There are other oddities, like firearms, gnome artificers, warforged soldiers who earned their 'freedom' by saving their gnomish creators and were discovered to be sentient...