What threat does the Prince make to Lord Montague and Lord Capulet in Romeo and Juliet?
I assume you are refering to the opening scene of the play, which features a brawl between various henchman from both of the feuding houses. We can see that the Prince is absolutely exasperated by this continuing conflict that appears to have no end. The Prince arrives just as the two Lords of these houses are about to fight together, and responds with anger. Note what he says to them:
If ever you disturb our streets again,
Your lives shall pay the forfeit of the peace.
Thus the Price tells them that the next person to disturb the peace in Verona will face death as punishment. Note of course how this becomes incredibly important to the plot when Romeo kills Tybalt, as it means he has broken the Prince's law and therefore is open to punishment.