The key question is: What will the US do about it? One step will be to leave no doubt about the US commitment to deterrence and US willingness to respond to any North Korean nuclear blackmail or use of such a weapon. The US must make it clear that it will deploy a decisively destructive nuclear option to strike North Korea — one capable of effectively destroying the country.
It must provide convincing “extended” nuclear deterrence to cover South Korea and Japan, and may well have to decide to either reintroduce modern US theater nuclear weapons, or smaller nuclear weapons designed to be effective at battlefield targets, to South Korea, or accept a South Korean nuclear force.
There must be no possible seam in the structure of deterrence, and both North Korea and its biggest trading partner China must fully understand just how strong the US retaliatory commitment is.