“Reign of death” is not a standard expression with a fixed meaning. It could be paraphrased as “rule of death”. I interpret it to mean something like “extremely brutal and violent regime”. Compare with the expression “reign of terror”=a period of time when a ruler controls people in a violent and cruel way. http://dictionary.cambridge.org/dictionary/english/a-reign-of-terror?q=reign+of+terror "soon we will drop our reign of death directly to your doorstep." is a joke - not a very good one, though (in my humble opinion). North Korea just had a missile test that failed. So the US population won’t have to worry that the “reign of death”=missile is going to be dropped on their doorsteps.