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.
"Sir Alec won't try anything. I've seen to it that half the city knows about the duel. We'll have an audience," says Lynette. You secretly drink the contents of the horn. You feel invincible! You reach the appointed place. Sir Alec is there, (DM rolls 1d20: 1!) looking confident. A dozen nobles lounge around the clearing. The Hon. Percy winks at you. "The principals are assembled! Shall we begin the duel?"
"Yes, there's no time like the present."
(I must out the traitors as part of this) "M'lady - shout your allegience to the Duke to all assembled and then let us begin!"