Located inside the Basilica of Sant'Eustorgio, Portinari Chapel is an outstanding example of Renaissance architecture in Milan, replete with a handsome dome, an ornate marble sepulcher adorned in bas-relief, and rich frescoes that include... a depiction of Mary and Jesus with devil horns? Indeed. But just like the story that inspired the fresco, this image is not quite what it seems.

Portinari Chapel was built from 1462 to 1468 and was consecrated to St. Peter of Verona, whose head rests in the elaborate marble shrine that serves as the chapel's focal point. Also known as St. Peter Martyr, Peter of Verona was a Dominican friar who spearheaded the Inquisition in northern Italy in the early 13th century, at the height of the Catholic Church's persecution of Cathars. He was famous for his persuasive sermons and proselytizing among the Cathars of Lombardy and neighboring regions.

One such story involved St. Peter of Verona seeking to "win back" an eminent man who had converted to Catharism after seeing the Madonna at a Cathar meeting. Determined to get to the bottom of this, St. Peter attended a meeting of the same group and saw the holy mother and child but—thanks to his unerring faith—saw also that the bore horns, revealing that this was actually the devil in disguise. Thus, he offered the false apparition a piece of sacramental bread, saying, "If you are the Mother of God, adore your Son!" The devil fled and as a result, all the Cathars present returned to Catholicism.

The frescoes of this and the other events of the life of St. Peter of Verona were painted by Vincenzo Foppa. After years of neglect, they were rediscovered in the late 19th century, and restored in the early 20th century.