Who is Merlin from That Hideous Strength and what is their importance?

Merlin does not come into the novel until about halfway through. It seems he has been sleeping in a sort of underground cave for centuries, and that the evil eldils who control the N.I.C.E. knew about his presence. The plot of the N.I.C.E. is to find him when he wakes and use his powers to destroy most of humanity. However, upon waking, Merlin goes to the house at St. Anne's and aligns himself with their cause. He is a man who is marked by characteristics from a different time.