Ambrosio and Matilda are brought before the Inquisition. Both proclaim their innocence, which means they must submit to the tortures. Matilda then confesses her guilt and is condemned to be burned at the Auto de Fe.
Ambrosio insists upon his innocence and is tortured. When returned to his cell to regain his strength for a second "questioning", he is visited by a vision of Matilda, who tries to convince him to completely yield his soul to Satan as she has. She leaves him the volume by which the rite is performed.
Ambrosio faces the Inquisition again. He again proclaims his innocence, but when faced with the instruments of torture once again, he admits to his sins of rape, murder, and sorcery. He too is condemned to burn. In despair, he calls up the image of Lucifer. Ambrosio requests Lucifer to save his life, who tells him it will be at the cost of his soul. Yet still Ambrosio resists, hoping eventually for God’s pardon. Lucifer informs him that there is none, and Ambrosio, after much resistance, signs the contract. He is rescued from the cell by Lucifer and brought to a wilderness.
Lucifer informs him that Elvira was his sister, making Antonia his niece, adding to his crimes the sin of incest. Lucifer reveals that it has long been his plan to gain Ambrosio’s soul, and Matilda was his servant in the process. Lucifer then carries Ambrosio up and drops him on the rocks below. Ambrosio suffers for six days, dying alone and damned for eternity.