La conversione e morte di San Guglielmo (The Conversion and Death of Saint William) is a sacred musical drama (dramma sacro) in three parts by the Italian composer Giovanni Battista Pergolesi.

[4] "The following summer Pergolesi was asked to set to music, as the final exercise of his studies, a dramma sacro in three acts by Ignazio Mancini, Li prodigi della divina grazia nella conversione e morte di san Guglielmo duca d’Aquitania ["The Miracles of Divine Grace in the Conversion and Death of Saint William, Duke of Aquitaine"].

[6] The autograph score of San Guglielmo has not survived but the various manuscripts which have been rediscovered show that the drama enjoyed widespread popularity for several years, and not only in Naples: in 1742 it was even revived in Rome, although only as an oratorio with the comic elements of the original removed.

[11] If the inspiration for this type of drama could partly be found in the autos sacramentales[12] and the comedias de santos[13] introduced to Naples during the era of Spanish domination (1559–1713) it was also in some ways a continuation of the old popular tradition of the sacra rappresantazione (a sort of "mystery play"), meaning "traditional religiosity, saturated in popular hagiography and rather naive in taste", which already featured comic characters expressing themselves in dialect, continual disguises and the inevitable conversion of the protagonists.

[9] Drammi sacri were not intended for the theatres but for places connected with houses of worship such as cloisters or parvises, or even for the courtyards of noblemen's palaces, and they were generally produced by the conservatories: students were called on to take part either in their composition or performance.

The conflation of these three figures is the result of a hagiographical tradition which was already well established by the time the libretto was written[16] and was not Mancini's invention.There are seven characters in the drama, five of them – those of a more elevated social rank and with more spiritual characteristics – are entrusted to higher voices (sopranos).

The remaining two roles, that of the plebeian captain Cuòsemo – a soldier in the entourage of the Duke of Aquitaine who follows his master's path to salvation, although reluctantly – and that of the Devil, are entrusted to bass voices.

In the score in the Biblioteca Giovanni Canna in Casale Monferrato, which follows the libretto of the oratorio version published in Rome in 1742 to the letter, there is also a third aria for Guglielmo, which would "restore the balance" in favour of the theoretically most important character but, according to Catalucci and Maestri, it is difficult to fit this piece into the dramatic action of the Naples manuscript.

Of course, the musical model adopted by the novice composer is twofold and consists in drastically differentiating the arias for serious characters from those for the comic ones: the former are virtuoso and da capo arias, sometimes full of pathos, sometimes bursting with pride, sometimes sententious, sometimes dramatic, most of them in a 'parlando' style, so as to create some dynamism in the development of the plot; the latter are syllabic, rich in repetitions, monosyllables and onomatopoeic words.Some of the music also appears in the score of L'Olimpiade, the opera seria Pergolesi composed for Rome just over three years later.

Whatever the case, the duet, in the version drawn from L'Olimpiade, was "highly celebrated throughout the 18th century"[23] and Jean-Jacques Rousseau made it the archetypal example of the form in the article "Duo" in his Dictionnaire de musique (1767).

Captain Cuòsemo, who has followed his master on his path to repentance and on his pilgrimage, begs the fake hermit for a morsel of bread but receives only contempt and refusal (duet, "Chi fa bene?").

A second intervention by the angel unmasks the demon again and chases him off, leaving Cuòsemo to curse him (aria, "Se n'era venuto lo tristo forfante"), before continuing his quest again.

William has been blinded in the battle and, overwhelmed by a sense of sin, finds himself plunged into despair once more (accompanied recitative and aria, "È dover che le luci… Manca la guida al piè"),[25] but the angel intervenes and restores his sight so he can "gather...companions and imitators".

The devil is horrified, but Alberto reveals his desire to join the monastery himself, while Cuòsemo offers a colourful description of the hunger and hardships which characterise monastic life (aria, "Veat'isso!

The angel greets William's soul as it flies up to heaven while the devil plunges back to hell vowing to return with renewed fury to continue his campaign of damnation (duet, "Vola al ciel, anima bella").