Bec lived in the time of the High King Diarmait mac Cerbaill (the mid to late 6th century). Diarmait had killed his wife's lover Flann mac Díma by burning down his fortress. For this deed, according to the legend, St. Ronan prophesied that Diarmait would die when a roofbeam fell on his head; Saint Ciaran prophesied that he would die in the same manner as Flann; but Bec mac Dé, Diarmait's druid, gave a much more elaborate prophesy, foretelling a threefold death. He would be drowned, burned, and have his head crushed by a roofbeam, by Flann's kinsman Áed Dub mac Suibni, in the house of Banbán the hospitaller, on a night that he wore a shirt grown from a single flax seed, drank ale brewed from a single grain of corn, and ate pork from a sow that had never farrowed.
Diarmait dismissed these prophesies, even when invited to a feast by Banbán. His wife, who had been listening, refused to go, so Banbán offered the king his own daughter for that night, and gave him a nightshirt, pork and ale. When the girl told him the nightshirt had been woven from flax grown from a single seed, the ale had been brewed from a single grain of corn, and the pork had come from a sow that had never farrowed, Diarmait realised the prophesy had been fulfilled, but too late. He tried to escape, but Áed Dubh was standing at the door with his spear. As Diarmait fled back into the house, Áed's men set it on fire. Diarmait tried to escape the conflagration by leaping into a vat of ale, but a flaming roofbeam fell on his head, and the prophesy was fulfilled.