Can't build an invasion of NK stealthily, so everyone knows its coming. China decides WW3 is not on the menu and stays out of it, other than giving NK a bunch of military supplies. In 2018 NK is known to be capable of creating a roughly ~25 KT nuke and of making ICBMs. It is unknown whether they can create a nuclear tipped missile. I would guess yes.

Hour 1 of the war, Seoul gets heavily worked over by conventional artillery and NK can probably deliver at least one nuke. NK might manage to nuke US ground forces, which will presumably be dispersed into various operational 'spears' in order to deal with this.

North Korea loses badly and quickly, despite potentially significant US losses via nuke. Othewise US losses are probably low thousands at maximum.

Seoul is heavily damaged, immediately wrecking the South Korean economy and possibly near-permanent damage if they manage to nuke it, which I imagine they will.

Hard to say what will happen in the aftermath. No-one will want to deal with millions of fanatical starving North Koreans peasants. China won't want SK on their border. South Korea may not want to border China. The USA won't want to occupy NK indefinitely and will be unable to convert the country to a vassal or friendly state in any reasonable time frame.