Symbolism, Imagery, Allegory
As Brachiano is dying from poison, Lodovico and Gasparo, disguised as monks, pretend to administer extreme unction (part of the last rites) to him. But, since the sacrament is a fake, it makes sure that Brachiano won't just die—he'll be damned for his sins:
Lodo. … He is departing: pray stand all apart,
And let us only whisper in his ears
Some private meditations, which our order
Permits you not to hear.
[Here, the rest being departed, Lodovico and Gasparo discover themselves.
Gas. Brachiano.
Lodo. Devil Brachiano, thou art damn'd.
Gas. Perpetually. (5.3)
In the same way that the sacrament is a fraud, Brachiano's outward mask of innocence has been a total fraud as well. He's only getting the kind of forgiveness his life demanded—false forgiveness. If he'd actually confessed to his murder plot or shown a slight sense of human remorse, maybe things would be different. But, given Brachiano's character, that probably wasn't going to happen.