The Marquis sees his ring on the Devil's finger
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As Griselidis ran down to the pirate ship her husband returned.
Still in disguise, the Devil tried to convince the Marquis that she was going to see a lover.
The Marquis spotted his own ring on the stranger's hand. He had given it to the Devil to seal their wager.
The Marquis realised that the Devil was trying to trick him.