The Korean War never formally ended. NK is taking increasingly hardline steps, according to most sources, in an effort to instil a sense of crisis in domestic populations.
In the event that a nuclear weapon was used, it'd almost certainly be of a tactical sort (due to their limited proficiency, especially in delivery system), which is nothing to sneeze at. However, I would doubt that the united states would respond by turning the entirety of the State into a glowing crater, for lots of reasons, but foremost because permanently fucking up the peninsula isn't a good thing. I think that seeing an attack on a SK/US troop concentration much more likely.
Casualty-wise, you're almost going to certainly see fairly heavy causalities amongst Republic of Korea troops as well as the US troops currently there, shortly followed by utterly disastrous causalities amongst PRK troops as superior air power, technology, reinforcements, artillery, ect. ect. ect. come into play.
Unknown numbers of civilian death, but almost certainly noticeable, especially in early stage shelling. PRK arty ain't known for accuracy.
Exactly what effect NK commando forces would have is pretty tough to guess, but I'd guess that the RoK would be up to policing the country, at least after the first shock is over.
And finally, there'd be a country of very impoverished, very unskilled workers to try to absorb into the region, which would destabilize both China and South Korea to heck and back.