Smirking, you hand Sir Alec the real horn. He takes a swallow and throws it back at you. "Thief! Murderer!" he shouts. Lady Lynette interrupts, "Come, sir, I am waiting. On guard." Sir Alec raises his sword shakily, and then turns and runs. Lynette lopes after him and (DM rolls 1d20+6: 12!) (DM rolls 1d8+4: 6!) wounds him in the side. Alec turns and (DM rolls 1d20: 20!) (DM rolls 2d8: 9!) gets a lucky gash on Lynette's leg. He tries to run past you.
"Fight as a man or fall as a coward Sir Alec, the crowd awaits your answer!" (I think the previous comment was to hand back the false horn, no bonuses for Sir Alec?)
Stick out my foot and trip him. "For shame Sir Alec! Fight now or be known to all assembled here as a coward!"
Ignore Sir Alec, for the moment, Lady Lynette is my real target here.
Lady Lynette draws her sword and says, "With all my heart. Duke Maxen, my liege lord, first and always!" Half the crowd echoes the sentiment. "I'll drink to that," drawls Sir Alec. He uncorks the fake drinking horn, shakes it, and then stares at it in horror. "What is this? Whose idea of a joke is this?" He hurls it to the ground.
Wait for the fight to end and then arrest the victor for treason.
fill the horn with water: "Is there something wrong with it?" and give it back.
Ignore Sir Alec and continue!
Snicker at Sir Alec and continue.
We can't arrest anyone for treason (#1) for fighting a duel, merely eliminate the Duke's supporters. Lady Lynette and Sir Alec were told directly by the Duke not to fight, so let them kill each other and be done with.