A German priest, Peter of Prague, troubled by doubts about whether the consecrated host was truly the Body of Christ, made a pilgrimage to Rome to pray at the tomb of St. Peter. On the way back, he stopped at the town of Bolsena and celebrated Mass in the crypt of the Church of St. Christina.
At the moment of the consecration, the host began to bleed onto the corporal (the linen altar cloth). The priest, overcome, wrapped the host and corporal and went immediately to nearby Orvieto, where Pope Urban IV was then residing.
Pope Urban received him, examined the relic, and in 1264 issued the Bull Transiturus de hoc mundo, extending the feast of Corpus Christi to the universal Church. He commissioned St. Thomas Aquinas to compose the office and hymns for the feast — Pange lingua, Tantum ergo, Lauda Sion, Adoro te devote — among the greatest Eucharistic poetry ever written.
The blood-stained corporal is preserved in a reliquary in Orvieto Cathedral, where it has been venerated for over 750 years.