A diabolical nuclear weapon which consists of a fission device encased in a jacket of cobalt.
When the fission occurs , fast neutrons turn the cobalt into a deadly cocktail of radioactive substances like Strontium-90 etc. which have an extremely long half-life. The resulting fallout will render the surrounding area uninhabitable for generations.

There is quite a high likelihood of such devices actually existing in the arsenals of many governments.