After making love to Matilda, Ambrosio experiences shame and disgust. He bitterly blames Matilda for tempting him to sin. Ambrosio discovers, however, that his lust is too strong to resist, and he continues his affair with Matilda.

Having won Ambrosio, Matilda resolves not to die of the snake’s poison but to employ her knowledge of sorcery (gained from her uncle) to save her life. Ambrosio is stunned by Matilda’s transformation from the timid and obedient Rosario; she now appears determined, ruthless, and commanding. To perform her ceremony, Matilda enters the “subterraneous Vaults” of the underground crypt of the Convent of St. Clare. Ambrosio accompanies her, but Matilda forbids him from witnessing her magic; he waits above while Matilda descends into the crypt. An earthquake, thunder, and flashes of lightning terrify Ambrosio while Matilda performs her rites, but he keeps his promise. Matilda emerges from the crypt cured and euphoric, describing her sorcery as providing her with “joys inexpressible.”

Ambrosio and Matilda continue their secret affair, but Ambrosio grows increasingly disgusted with Matilda. One night, Antonia appears at the church when Ambrosio is hearing confessions, and she begs Ambrosio to visit her mother, who is gravely ill.