A gala celebrated the opening of “Sade: Marquis of the Shadows, Prince of the Enlightment” at the Museum of Letters and Manuscripts. Tomas van Houtryve The Count de Sade, the modern descendant of the Marquis de Sade, whose rabid erotic works inspired the term sadism for sexual cruelty, resides in a sunny and strikingly decorated apartment on a quiet residential street on the Right Bank of Paris. A