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According to legend the Cornu was a great black bird, described as being like a wingless and featherless heron that occupied a cavern at the medieval pilgrimage site of St Patrick’s Purgatory, Saint’s Island, Lough Derg, where it is said to have “uttered a cry like a blowing of a trumpet, which presaged the death of some pilgrims”.
The Cornu is said to have been a daemon (daimon, demon) that was transformed into a bird by St Patrick and exiled to Lough Derg. According to Giraldus Cambrensis (Gerald of Wales) (born 1146 – died 1223)in Topography of Ireland:
“THERE is a lake in Ulster containing an island divided into two parts. In one of these stands a church of especial sanctity, and it is most agreeable and delightful, as well as beyond measure glorious for the visitations of angels and the multitude of the saints who visibly frequent it. The other part, being covered with rugged crags, is reported to be the resort of devils only, and to be almost always the theatre on which crowds of evil spirits visibly perform their rites. This part of the island contains nine pits, and should any one perchance venture to spend the night in one of them (which has been done, we know, at times, by some rash men), he is immediately seized by the malignant spirits, who so severely torture him during the whole night, inflicting on him such unutterable sufferings by fire and water, and other torments of various kinds, that when morning comes scarcely any spark of life is found left in his wretched body. It is said that any one who has once submitted to these torments as a penance imposed upon him, will not afterwards undergo the pains of hell, unless he commit some sin of a deeper dye.”
It has been suggested that the Cornu lived in this evil half of Saint’s Island, in the northwest, which has been referred to as Kernagh (Island of Clamour).