The term, decorum, means dignified or proper behavior by example. It is used in the story as "...but the figure in question had out-Heroded Herod and gone beyond the bounds of even the prince's indefinite decorum."

This is in reference to the outlandish and ghoulish way in which the stranger was dressed during the costume ball. The fact that he had offended the prince, who was throwing a ball while many are dying form the plague, shows just how disturbing the image was.

The figure, is death, coming for those who thought they had manged to escape it by holing up with the prince.