I think the missi dominici were men sent out by (originally) Charlemagne. They were sent out to make sure the rulers under him were carrying out his will.
Missi dominici was the name given to the officials commissioned by the Frankish kings and emperors to supervise the administration of their domains. Their institution dates from Charles Martel and Pippin the Short, who sent out officials to see to it that their orders were executed. When Pippin became king in 754 he occasionally sent out missi; but Charlemagne made them a regular part of his administration.