The works of the Marquis de Sade were suppressed by the mid-19th century, but were floating around Europe in samizdat form until the mid-20th century, when they were published and circulated above ground. These semi-pornographic works promote a philosophy of negative Christianity, but invert the Christian ideas of morality and find pleasure in cruelty and naked power.
How much of a debt does Nietzsche's philosophy of slave/master-morality owe to Sade? Is there any credible evidence that he borrowed aspects of his philosophy from Sade's work, or was even exposed to this writing? If so, what aspects of his work as we know it are original to Nietzsche, and what parts are due to Sade's influence?
I suspect that the scholarly research devoted to this question is limited because Sade was not available until the end of World War II, and even today, there are those who will not read him. Some writers credit Sade with inventing the modern villain, the gothic novel, and the horror genre; in fact, during Sade's time, he was considered a religious writer since, like Film Noir, he paints God in negative space.