A very deadly disease breaks out killing most of the population.
All of the very "important people" (including the prince), hid in the castle away from the sick people.
The people inside the kingdom throw a masquerade, during the masquerade a clock goes off in the scariest of the seven rooms scaring the guest (this happens twice).
A man enters the great hall to join the party. His costume was offending to the "important people" who thought it was related to the deadly disease outside.
The masked figure immediately leaves the great hall when the prince demands he be captured. When all is still and the guards don't move an inch, the prince charges after the masked man with a dagger.
The prince catches up to the masked man. The masked man grabbed the princes dagger and stabbed the prince. When the guards captured and unmasked the princes killer, they found nothing but death itself.