Well the basic is
The world started as modern day
Then genetic modification came in and changed what people were
[magic back story redacted]
Then world war broke out and elves and red guards became mass produced modified versions because they were really well used for war.
((I use the elder scroll names for ease of use and no that's not his problem, we had a rather long winded discussion of it on the old game))
Then nuke strikes were used because ground war wasn't working.
Everyone retreated into fallout bunkers for several hundred years.
The great river is one of many fall out bunker concentration zones because of it's abundance of large mountains.
The bunkers open up and nomadic tribes of survivors begin pouring out.
Social strain and racial problems strike up causing people to identify within one of three races [elves-humans-red guards] elves consisting of the three elven races, humans consisting of humans, argonians, and Khajiit and later in history the children of the wilds but they are a bit of human and elf, the red guard races consisted of red guards and dwarves and later in history the orcs.
The nomadic tribes began to converge as villages and then became their nations, this is where the game starts off
The tech is medevile except for the solar powered lights and radio comm systems that survived the release from the bunkers