The Miracle at Bolsena immediately comes to mind, of course (Eucharist spouted Christ's blood, mid-13th c); reliquary is in Orvieto Cathedral. The miracle itself took place at S Cristina in Bolsena. If you look up "Orvieto" in the Catholic Encyclopedia, there are a couple of paragraphs on the subject. Here's a recent essay on the reliquary:

Author: Freni, Giovanni.

Title: “The reliquary of the Holy Corporal in the cathedral of Orvieto : patronage and politics.”

In: Art, politics, and civic religion in Central Italy, 1261-1352 : essays by postgraduate students at the Courtauld Institute of Art. Aldershot, Brookfield, Ashgate, 2000, p. 117-177.

Summary: Cites documentary evidence and financial records for the commission and execution of the reliquary by Ugolino di Vieri and his workshop, finished in 1338. Tramo Mondaleschi della Cervara, bishop of the city of Orvieto, through the Opera del Duomo, commissioned the new and magnificent reliquary for the relics of the "miracle of Bolsena". The leading role played by the powerful Monaldeschi family in this commission is supported by Tramo's inscription on the base of the reliquary and further confimed by the presence of coats of arms, together with those of the pope, in a central position between the shields with the emblems of the city. The importance of the feast of Corpus Christi at this time, and of the cult of the relics of the miracle of Bolsena, prompted the commission. The narrative cycles of the reliquary were designed to provide a visual exposition of the doctrine of the Corpus Christi through the exemplum of the "Miracle of Bolsena" and the narrative of significant episodes of the Gospels. Religious and civic spheres were inextricably interlocked in the plans for the promotion and display of Orvieto's precious relic.

-Patricia