The plot of Là-Bas concerns the novelist Durtal, who is disgusted by the emptiness and vulgarity of the modern world. He seeks relief by turning to the study of the Middle Ages and begins to research the life of the notorious 15th-century child-murderer Gilles de Rais. Through his contacts in Paris (notably Dr. Johannes, modeled after Joseph-Antoine Boullan), Durtal finds out that Satanism is not simply a thing of the past but alive in turn of the century France. He embarks on an investigation of the occult underworld with the help of his lover Madame Chantelouve. The novel culminates with a description of a black mass. (Wikipedia)
Reader's note: This novel certainly reaches to the depths of depravity and blasphemy, and achieved some notoriety for doing so. A redeeming feature, if one is required, is that Durtal begins in this book a pilgrimage; like Dante's hero, he begins this in a horrible and God-forsaken place.