Based on an old celtic god that was really worshipped in the past
Crom Cruach is a Celtic pagan deity that resides in a cave within The Forest outside of Kells, and most likely other, similar dwellings. He killed Aisling's mother and the rest of her people. Aidan's predecessor, Collum-Cille was once in posession of one of the God's eyes, The Eye of Collum-Cille, a special lense that works like a very good magnification glass and is required to finish the Book of Iona. Brendan and Aisling go to the Cave of Crom Cruach to claim the god's remaining eye after Brother Aidan told Brendan that the Northmen have destroyed the first one, so they may finish the book. Crom nearly kills Brendan, but he finds a way to fight him; he uses a piece of chalk to "draw" a kind of prison for the god and eventually defeats Crom by taking out the his remaining eye so that the blinded Crom bites pieces out of his own body in order to catch Brendan whom he can no longer locate and eats himself up completely.