Image of St Primianus that is taken in

procession in Larino each 15th May

A decree (1067) by Bishop Andrea II reveals that there was a structure near the old Duomo known as the "trivuna Sancti Primiani".   This was probably the apse of what had been a church dedicated to St Primianus.  In the early 20th century, Giuseppe Sordini discovered a structure under what is now the Cappelle delle Reliquie of the present Duomo that is generally accepted to be the crypt of this church.  Excavation  has revealed that the crypt was probably built in the 9th century in the form of an annulus, almost certainly to facilitate access to the relics of a martyr, presumably those of St Primianus. 

According to a tradition, St Primianus was martyred in Spoleto on 31st August in the reign of the Emperor Diocletian (ca. 303).  However, the fact that this tradition can only be traced back to  the 16th century leaves its veracity in doubt.  A more likely explanation is that relics of St Primianus were brought to Spoleto in or before the 9th century, in which case the feast on 31st August might have commemorated the translation.

If this is broadly correct, there are at least two candidates for the identity of the saint in question:

1. ✴St Primianus of Larino (28th April and 15th May)
   

2. According the the legend BHL 6921d, St Primianus was martyred on 15th May, 303, along with his brothers Firmianus and Castus, in the ancient amphitheatre of Lerinum (Larino), a town in what became the Lombard Duchy of Benevento.  The church of San Primiano in the cemetery there stands on the site of an early Christian church that may well have been built to house the relics of the martyred brothers.  Saracen mercenaries destroyed Larino in 842.  They were probably fighting in the civil war that engulfed the Duchy of Benevento after the murder of Duke Sicard in 839.  The relics were stolen during the chaos and taken to nearby Lesina: the feast day (28th April) marks the date of this translation.  The relics of SS Primiano and Firmianus were rediscovered in the crypt of the cathedral of Lesina in 1598 and taken to church of SS Annunziata di Napoli for safety.  They were returned to Lesina in 2000.
   

3. ✴St Primianus of Ancona (23rd February)
   

4. The existence of the relics of this saint in the church of Maria in Turriano, Ancona was revealed to a lady during a dream in 1376.  They were subsequently found there, together with an inscription: "Hic requiescit corpus primiani episcopi et martyris qui fuit graecus" (Here lies the body of the martyred Bishop Primianus, who came from Greece).  The relics was destroyed during bombing in 1943.
   

The first of these is the better candidate, since Duke Guy I of Spoleto was very much involved in the civil war in the Duchy of Benevento in the 840s.  (For the historical background, see the page on the Carolingian Duchy of Spoleto).  He may well have secured some of the relics during the chaos at Larino in 842 and taken them to Spoleto.  If this is the case, the date of 31st August probably records their translation to Spoleto.

When the old high altar of the Duomo was demolished in 1653, a number of relics were recovered that were believed to include the cranium of St Primianus.