He had married the French spy Helene. He was rich, an excellent swordsman, and not without some influence unfortunately he was to be a pawn in a plot devised by Pierre Ducos.
Helene was ordered to write her husband about how she suffered under the unwanted attentions of Richard Sharpe. The Marquese immediately set about challenging Sharpe Sharpe to a duel over the matter of Helene's honor.
He is a far more skilled swordsman than Sharpe, but skill is nothing compared to the full force of an angry Sharpe, but the duel is broken up before Sharpe can land the killing blow.
Since his survival is counter to their plans the partisan, known as the El Matarife and his brother the Inquisitor priest, Father Hacha arranged for his death that night. El Matarife slashed his throat while he slept, and Sharpe is accused of the murder.