formal if you are told to do something on pain of death, you will be killed if you do not do it.
They had been told to leave their homes by noon on pain of death.
under pain of death
With the threat of dying or being executed as a punishment. (Sometimes used hyperbolically.) The prisoners were forced to march under pain of death from the POW camp across the burning desert.I am under strict instructions by my wife not to leave the house in disarray by the time she comes back from her trip, under pain of death!