PARIS -- "As soon as the grave is filled in, acorns should be planted over it, so that new trees will grow out of it later, and the wood will be as thick as it was before. All traces of my grave shall vanish from the face of the earth, as I flatter myself that my memory will vanish from the minds of men."

From the 1806 will of the Marquis de Sade

The narrow figure moves along the marble-columned corridor of France's National Assembly. A finger points, a name is uttered, a uniformed guard turns to his partner.

"Is he any relation to the ..."

"The great-great-great-grandson," replies the partner, gravely.

Eyes follow the figure until it disappears.

Silence. Then, a whisper: "Mon Dieu! "

Living up to the family name is anyone's challenge, even more so when lineage claims an ancestor of fame, or infamy.

"It's always the name," says Thibault de Sade, who is himself a marquis, and an attorney, an assembly legal counsel. "A little spark just goes off in people's eyes. First, they're puzzled. Usually they wait a little before asking, 'You're not by any chance ...' Or 'You wouldn't be ...' "

The sentence invariably goes unfinished.

"They never come outright and ask."

Slight, strikingly pale and angular, a straight-edge razor in a dark suit, Sade, 33, has spent the last decade researching a biography that he hopes will redeem, or at least soften, his ancestor's scandalous reputation. The timing is auspicious. This is the 250th anniversary of The Marquis' birth.

To be published here in the fall, "Sade" discloses the existence of an unknown romantic trilogy, and of an astonishing historic curiosity: an opera written by Sade about -- and once performed by -- inmates of the madhouse where he was held during the French Revolution.

This Marquis de Sade is preparing the play for production.

"Its discovery," says Thibault mysteriously, his eyes shining like two black stones, "reads a little bit like a detective story."

Few names, in any century, through distinction of act or accident, achieve lasting inclusion in the language of the day. In this century, "Churchillian" has come to describe a certain demeanor or stature, and "Freudian" attaches itself to a panoply of libidinous disorders.

Neither holds a candle to the enduring Pandora's box called Sade.

Used as a noun, as an adjective or as a prefix, the name in all its declensions has come to be synonymous with decadence and debauchery, with the pathological confusion of pleasure and pain, with the exercise of debasement as a form of sexual gratification, with flogging and spanking and raping, with licentiousness and cruelty in all forms.

All this is the ignoble legacy of a nobleman, the Marquis Donatien Alphonse Francois de Sade (1740-1814), a Frenchman whose degenerate behavior and prurient writings kept him in prison for 27 years, much of that time sentenced to death. It was there -- frustrated to near delirium by his inability to satisfy his unnatural lusts -- that he wrote in crabbed, prissy longhand the three angry, anticlerical, violent, wildly uninhibited erotic classics for which he is best known: "120 Days of Sodom," "Juliette" and "Justine."

The books were published privately, purchased furtively, read alone. His power to shock persisted for centuries. In the 1800s girls were said to have gone mad or committed suicide after reading Sade's novels. To publish them remained a crime in France until the 1950s, and even afterward reading them was thought perilous to one's sanity. As late as 1966, Sade's writings were blamed for inspiring the torture-murder of three children by a British couple who described themselves as slavish devotees of the works of the Monstrous Marquis.

A black saint; a French satyr; a Jekyll-Hyde with whip and quill, Sade is a man without a country. There are no monuments to Sade in France. In this anniversary year, there are no official commemorations of his birth.

Such is the tarnished image Thibault de Sade seeks to polish.

It would be easier, perhaps, to polish a puddle of mud.

Consider the brief Sade entry in the biographical section of Webster's Seventh New Collegiate Dictionary, which thumbnails the complicated, controversial life of the decorated hero of the Seven Years' War, the champion of moderation during the infamous Reign of Terror, the political and intellectual revolutionary whose works -- for better or worse -- influenced eight generations of French literature, from Flaubert to Hugo to de Beauvoir, thusly:

"De Sade, Donatien Alphonse. French soldier and pervert."

End of entry.

Unraveling the Mystery

"The name still has tremendous power over people," acknowledges Thibault de Sade, gesturing grandly as he sits at The Bourbon, a Paris cafe. He has long, impossibly long, delicate hands like a pianist's, and masklike features suggesting a perpetual frown. His is a face of shadows, with recessed eyes, hooded brows. He bears an inescapable likeness to the figure depicted in the only known portrait of the dark marquis.

"He's so much a legend to people," says Thibault, his voice like thick paper. "A mythic symbol, like King Arthur or Merlin. Many people don't even think he existed."

There is good reason for this. Until the 1950s, as an official nonperson of French literature, Sade was little more than a clandestine whisper among intellectuals.

Until relatively recently, even the modern-day Sades hadn't heard of their invamous ancestor. Incredibly, Thibault de Sade is the first direct descendant of Sade in at least three generations to know the secret from birth.

Like his father and grandfather, Thibault was raised in a remote country village in the south of France, where "Sade" meant no more than a name on the chateau mailbox. The original marquis' own son, says Thibault, "struck all mention of him from family documents -- actually scratched out his place in the lineage. For generations, it was forbidden, a taboo in the family to speak of him. Eventually they hoped he would cease to exist."

He almost did, at least in the family history. Thibault learned of his lineage from his father, Xavier-Helene de Sade, who discovered it by accident, when he stumbled across a book containing a biography of his infamous relation. For Xavier-Helene, a scholarly country squire, it was the answer to a conundrum that had been bedeviling him since the end of World War II.

In 1947, when Xavier-Helene returned to his family estate at Conde-de-Briein after imprisonment in a Polish labor camp, he discovered to his horror that the house had been ransacked and looted by Nazi soldiers during their occupation.

Along with broken furniture and other detritus, there were piles and piles of ancient papers -- handwritten letters and monographs and manuscripts, yellow, brittle pages torn from bindings -- scattered all over.

Puzzled, Thibault's father brought the papers to his elderly uncles, wanting to know who this curious ancestor was who seemed to correspond regularly with Voltaire, Robespierre and Louis XVI, and who had these rather ... unharnessed views on sexuality. The uncles -- steeped in family history -- huffily told Xavier that they did not know. And furthermore, that it was better not to ask.

Once he learned from the biography he chanced upon just who the original marquis was (this is the beginning of Thibault's "detective story") Xavier-Helene set about meticulously reading and cataloguing the papers. Even as a youngster, Thibault joined him.

"He'd sit at breakfast," Thibault recalled, "stir his coffee and pull from a handful of papers at the table. Then he'd read it aloud and we'd discuss it. One morning there would be a letter describing confinement in the Bastille. Then Sade would be condemned to the guillotine. Then, months later, we'd discover his escape from prison.

"The little concerns of the Marquis' life began to pass like a pageant on paper. These papers were still all mixed up, but in one, he was discussing the new things of his time: bicycles and glasses. As a child of 12, this I understood.

"For me, it was like an adventure tale."

These documents formed the core of what would become Thibault's biography. He and his father were laboring in obscurity. It became the family's private obsession, but few others seemed to care.

"In school in the little village, it wasn't anything," Thibault says. The first time anyone expressed interest in his name, he says, was during his college entrance exams.

"It was a 90-minute oral in finance, and I'm not very good in that subject. The professor asked me about finance for five minutes, and about Sade for 125, and I got a very good grade. Otherwise, I wouldn't have passed."

The Making of a Monster

Little is known about Sade's early years that explains his later fascinations with sex and torture. It has been written, with dubious foundation, that as a child he once surreptitiously witnessed his aunt making love, and that when he was caught, was soundly thrashed; supposedly, forever after he came to associate sexual pleasure with pain. Apocryphal or not, the story has become part of Sade lore.

A landed aristocrat decorated for bravery after the Seven Years' War, Sade was sentenced to death in absentia in 1772 for "crimes of sexual cruelty." The charges involved a trip to Marseilles that Sade made with his manservant, during which the two procured the services of no fewer than five prostitutes. In an apparent effort to make the women amenable to sodomy, whipping and other indignities, Sade allegedly plied them with pastilles laced with a powerful aphrodisiac. When two of them nearly died of stomach poisoning, they went to the authorities.

Sade eventually was captured through the intervention of his mother-in-law, one Madame Montreuile, who would stalk him vindictively for the rest of her life.

Locked away in the Bastille in 1784 -- but excused from his death sentence -- Sade began his tormented, demented prison writings.

His intellectual arrogance was undiminished in prison; the man was no diplomat. Once, denied access to the works of Rousseau by prison officials who feared it would incite him, he scolded them in a letter: "Gentlemen, have the good sense to understand, in sending me the book I ask for, that Rousseau could be a danger to bigoted hypocrites like you, but is an excellent book for me."

In early July 1789, as the revolution was brewing, Sade fashioned himself a megaphone and began ranting hourly to passersby through a barred window. When he was denied his daily courtyard walk, he cried to the populace, "They're torturing the prisoners in here!" and called for the citizens to "storm" the prison. For this, he was transferred to the madhouse at Charenton, but his rantings had their effect. Ten days after he was removed from the Bastille, it was indeed "stormed," and the French Revolution began.

In January, granted a certificate of health, "Citizen Sade" was released from the asylum. Freed into the insanity that was the Reign of Terror, Sade emerged an influential member of the moderate faction, urging an end to the random guillotinings, even sparing the life of his personal bete noir, his mother-in-law.

Eventually, he fell into political disfavor. Declared "seditious" by Napoleon, he was reimprisoned for life, and his works forever banned.

Sade died alone in Charenton, grossly fat and mumblingly mad, on Dec. 2, 1814.

Six months later, Napoleon fell at Waterloo.

Misunderstood Marquis

Misunderstood, says Thibault de Sade. The marquis was tragically misunderstood.

Yes, many of his works were appallingly offensive, written to satisfy Sade's undeniably aberrant appetites. Throbbing with helpless heroines who want to be virtuous but are deflowered and debauched -- inventively -- by rapacious heroes, and with nuns and priests fornicating with abandon, Sade's prose still can move reasonable people to revulsion and anger.

But to judge him merely as a pornographer is to misunderstand the value of his work in the context of his times, Thibault says. Sade, he says, was writing about the inherent evil of mankind, and the hypocrisy practiced by the guardians of the public morality: His argument was not, primarily, for the practice of cruelty, but for the value -- above all else -- of freedom of expression and of thought.

Thibault is not alone in this assessment. The past quarter-century has seen, if not a total cleansing of Sade's reputation, something of a reassessment of thought about the Infernal Marquis, at least among intellectuals.

The late feminist Simone de Beauvoir noted that the only crime for which Sade was ever convicted, "the whipping of two local girls in exchange for a prearranged fee," was "a fairly petty offense" considering the penalty he received.

In France today, even independent of Thibault, there exists a growing urge to forgive. Sade, whose works fulminate with scenes of abuse of women, ironically has been embraced by France's rising feminist movement; he is analyzed, and largely praised, as a creator of beyond-the-conventional heroines. Writer Annie Le Brun, an outspoken Sade advocate, suggests that Juliette -- a harlot who willingly engages in orgiastic sex -- does "an amazing striptease in which she removes, one by one, the garments of 'femininity' which society seeks to force on her."

"Sade," she said, "excites us. He insists that freedom is dangerous. But it is also limitless. There is something in man that knows no limit and this he links to desire."

Last year, while official Bicentennial celebrations took little note of Sade, more plays appeared about him than all the other members of the Revolution. There were five separate Sade productions in Pairs alone.

For Thibault, that was good news. The bad news: in most of these, Sade is portrayed as a whip-wielding, swaggering, jackbooted...well, sadist.

Songs & Sidelines

Thibault's biography is based on the papers catalogued from the family chateau, and other documents he recovered, improbably, from the archives at the University of Kansas, where they had been donated by a private collector decades ago and forgotten. Taken together, they will increase by half the Marquis de Sade's total known writings -- and present, says Thibault, a more complete, and more human, portrait of the "monster."

"In most of his work," Thibault says, "you have the romantic writer, who doesn't inspire fear."

The centerpiece of the new material is clearly "Hommage a` la Reconnaissance" (A Tribute to Gratitude), the play that is the story of the madhouse fools at Charenton.

"It is Sade," says Thibault, "trying to show that sanity and madness are very close and nearly inseparable." The 70-page parchment libretto is dedicated in Sade's own hand, a peculiarly angled stroke, 'to the asylum's director, Monsieur Coulmiers.'

" 'The little opera,' " as Thibault calls it (discussing production plans for next year), "is comic on one hand; satiric on the other. And, as it's Sade, writing in the same year as 'Sodom' and 'Justine,' there's a certain sexuality."

With songs and comic dialogue, the work is cast with recognizable characters from the asylum, including Coulmiers, an enlightened warden who allowed his inmates books, music and theater; and Sade, who played himself and, evidently, sang in its performance at the asylum.

(A passing mention of this event in a previous biography, Thibault believes, was the inspiration for Peter Weiss's fictional 1964 play, "The Persecution and Assassination of Jean-Paul Marat as Performed by the Inmates of the Asylum at Charenton Under the Direction of the Marquis de Sade.")

These days, Thibault is a busy man. He is preparing the play, finishing the biography and branching out into a less artistic, and more commercial, exploitation of his infamous progenitor's celebrity.

Thibault is discussing the marketing a line of chocolates ("sinfully rich") and is alreading selling Champagne Marquis de Sade. Even the bubbly has proved controversial. The bottles were rejected for importation to America because they were deemed "pornographic" -- the label contained a small representation of a female nude.

Meanwhile, sales in Europe are going well.

The champagne, appropriately enough, is a "brut."

It Runs in the Family

Thibault de Sade is closing out his interview at the cafe, defending his upcoming biography against anticipated attack.

"For me, it's not a question of redemption -- making a devil into an angel. Surely, he was a little of both. For me, the biography is like a portrait. I want only to give people a frame and canvas, then let them step back and decide for themselves."

An unsmiling waiter drops the check on the table.

Moving quickly, Thibault slaps a reporter's reaching hand away with insistence. Meant playfully, nevertheless its polite sting raises a rush of blood, a warm -- but no, not an entirely unpleasant -- sensation across the back of the hand.

Thibault cautions, as he reaches for the check: "Never anger a Sade. It's dangerous."