Sant'Eustorgio, when he was still but a bishop, had this church built in the fourth century to house the relics of the Three Kings. The specific site was chosen when animals pulling the relic-laden cart reached this spot and refused to budge. In 1164, Frederick Barbarossa absconded with the relics, but they were returned (in part) in 1903 and are venerated at Epiphany (6 January). The ceremony takes place at the end of a mass timed to coincide with the arrival of a procession from piazza del Duomo led by the 'Three Kings' (played by a trio of Milanesi).
The church is filled with works by Milanese and Lombard artists from the 13th to the 15th centuries: Giovannino di Grassi, Giovanni da Milano, Giovanni di Balduccio (see his 1327 Gothic funeral monument to Stefano Visconti on the left wall of the fifth chapel on the right), Bernardino Luini and the maestri campionesi all feature. The fourth chapel on the right contains the 14th-century painted wooden crucifix that was supposed to cure pregnant women of fever.
The main attraction, however, is the Cappella Portinari next door, built between 1462 and 1466 by Florentine banker Pigello Portinari for his own tomb, and as a repository for the body of St Peter Martyr, murdered when heretics stuck a knife into his skull 200 years earlier. Perhaps the earliest truly Renaissance work in the city, the chapel unites the classical forms championed by Brunelleschi in Tuscany with typical Lombard fresco decoration by Vincenzo Foppa. Foppa's scenes of the life of the Virgin and of St Peter Martyr's miracles (1466-68) are perhaps the painter's greatest masterpieces. In the centre of the chapel, the intricate, carved Ark of St Peter Martyr, containing most of the saint's remains, is by Giovanni da Balduccio (1336-39); the rest of him (his skull) is in a silver urn in the little chapel to the left of the Cappella Portinari.
The chapel's entry price also allows you access to a room stocked with hundreds of saints' relics, housed in glass cabinets, and an oppressive but interesting Paleochristian cemetery, located under the Sant'Eustorgio Basilica.