Starting on the Tuesday before Pentecost of the year 1405, the Lord appeared covered with His wounds to John of Huldenberg, master of the place. Only at the third apparition did our Lord speak ordering John “Go into the Chapel of Isaac, you will find Me there.” At the same time, the parish priest, Peter Ost, heard a voice instructing him to offer the Mass of the Holy Cross in the Chapel of Isaac. The following day the pastor summoned all the faithful to assist at Mass at the Chapel of Isaac. John of Huldenberg was among those present.
The priest began the Mass, and when he unfolded the corporal he saw there a Particle of the large Host that had been consecrated the preceding Tuesday. He sought to receive the Host but the Eucharist clung to the corporal and began to Bleed. The priest turned white and John, who had observed everything, comforted him by saying: “Do not fear, this marvel comes from God”, and related his visions. For four days, the Blood continued to flow, that is until Tuesday after Pentecost, the Blood continued to flow, reaching the length of a finger in three breadths. Then, having stained the entire corporal, the Blood co-agulated little by little and dried up.
The Bishop of Cambrai, Peter d' Ailly, was informed of what happened and he decided to investigate personally and had the corporal in his care for some two years. Every attempt to remove the stains of Blood on the corporal were useless. The Bishop opened an investigation where testimonies were gathered regarding the prodigies wrought by the reliquary of the precious Blood. On June 16, 1410, the Bishop granted an indulgence of 40 days to those who visited the Chapel at Bois-Seigneur-Isaac.