It sounds like you are describing the first book of Terry Goodkind's The Sword of Truth series, Wizard's First Rule.
In the book, the antagonist, Darken Rahl, uses an extreme form of molybdomancy early in Chapter 24 (found online with a Google Books search):
While Carl did as he was told, Rahl chanted the last charm, his heart pounding. The torches hissed and spit while they burned, the sound intertwining with that of the spell.
And then he poured the contents of the crucible into the horn.
Carl's eyes snapped wide, and he both inhaled and swallowed involuntarily when the molten lead hit him, searing into his body.
The Wikipedia summary for the rest
Soon an artist who can control enchantments, separates Richard from the group and he falls into the hands of a Mord-Sith named Denna who brutally tortures him for a month. The Mord Sith report to Darken Rahl, and Rahl hopes to force Richard to recite the Book of Counted Shadows, a magical book which Richard had memorized under the bequest of his father. However, Richard's innate gentleness alters their relationship, and Richard eventually breaks free of Denna's control by turning the Sword of Truth white, with the power of love for her, rather than the sword's typical harness of anger.
The entry for Denna has more information:
Denna falls in love with Richard, due to his insistence on showing her compassion despite the way she treated him. She eventually asks him to kill her to free both himself from Darken Rahl's clutches and herself from her role as a Mord-Sith. She gives Richard her agiel as a token of her love right before he kills her.