Based on a Celtic god that was worshipped in the time before the middle ages.
Crom Cruach is a Celtic Pagan deity that resides in a strange darkness realm, the entrance to which is found in a cave within The Forest outside of Kells. He killed Aisling's parents and the rest of her people and seems to cause a weakening or toxic effect(her appearance decaying into that similar to a wilted flower) to her. Aidan and his predecessor, Collum-Cille was once in possession of one of the God's eyes, The Eye of Collum-Cille, a special lens that works like a very powerful 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. Crom get's ahold of his own tail and begins eating himself in a circle.