Modena is a town in Italy1, whilst an archivolt is a decorative moulding which is carried around an arched wall opening2. The construction of a new cathedral the Duomo di Modena, began at Modena in the year 1099, and above one particular arched doorway called the Porta della Pescheria3 is a carving in relief which has become known as the Modena Archivolt.
This shows a woman named as 'Winlogee', imprisoned in a tower by two characters, one labelled as 'Mardoc' and the other as 'Burmaltus', who are both being attacked by two knights named as 'Isdernus' and the other as 'Artus de Bretania', while a further knight shown as 'Carrado' is being charged by three knights labeled as 'Galvaginus', 'Galvariun', and 'Che'.
'Artus de Bretania' is naturally, Arthur of Britain or King Arthur as he is more commonly known, 'Winlogee' is Gwenhwyfar, or Guinevere in the English, 'Mardoc' is Mordred, 'Carrado' is Caradoc, Galvaginus is Gawain etc, all familiar figures from Arthurian mythology.
What the Modena archivolt is depicting is the tale known as The Abduction of Gwenhwyfar (or Guinevere if you prefer) which was first recorded in Caradog of Llancarfan's Vita Gildae or Life of Gildas, which is believed to have been written in the 1120s or 1130s for the monks of Glastonbury.
This is a tale of a king known as 'Melvas' or 'Melwas' who kidnapped Gwenhwyfar, had his wicked way with her, and then took her back to his capital at Glastonbury4. Caradog of Llancarfan writes that Melwas was besieged there "by the tyrant Arthur with a countless multitude on account of his wife Gwenhwyfar, whom the aforesaid wicked king had violated and carried off, and brought there for protection". It is then that the abbot of Glastonbury steps in to act as an intermediary, reconciling the two kings and restoring peace to the land once more.
Since the evidence points to the sculpture probably being carved between the year 1100 and 1125 (and therefore predates the publication of Geoffrey of Monmouth's infamous Historia Regum Britanniae in 1136), it ranks as the very first surviving depiction of king Arthur and is important because it demonstrates how the myths surrounding that particular gentleman was beginning to spread around Europe even before Geoffrey of Monmouth had put pen to paper. And Caradog of Llancarfan's work is itself too late to explain how a bunch of Italian scupltors knew the same story.
No one knows quite how the story made its way from Wales (or Britanny quite possibly, as many of the same stories circulated there), or quite why this happened in the twelfth century. But it probably has something to do with; first of all the Norman conquest of England in 1066, which dragged England, and Wales with it, back into the mainstream of continental European culture, and secondly with the First Crusade of 1095-1099 which resulted in parties of crusaders travelling through Italy enroute to the Holy Land (and back again). (All of whom were no doubt, busily swapping stories as they went to help pass the time.)
NOTES
1 Specifically in north central Italy, basically half wat between Genoa and Venice; famous for balsamic vinegar and Ferrari motor cars.
2In the same way that an architrave is a decorative moulding which is carried around a square wall opening.
3 Which rather prosaically translates as the 'Door of the Fish market'.
4 Although not entirely unwillingly as many versions of the tale suggest.
SOURCES
http://www.ithaca.edu/faculty/twomey/sggk/legend.html
http://www.users.globalnet.co.uk/~tomgreen/figures.htm
The Modena Archivolt can be viewed online at;
http://camelot.celtic-twilight.com/sgmaoa/sculpture1.htm and
http://www.scsr.nevada.edu/~westrad/medLit/pages/modenavolt.html