Similarities with Tolkien's worksEdit
There are some similar themes between the major works of the two writers, which might be no coincidence - since they are known to have, while writing their respective books, shared their "work in progress" with each other and had no objection to the other "borrowing". In the preface to "That Hideous Strength" Lewis made explicit reference to Tolkien's Numenor, which Lewis spelled "Numinor" and linked it with the plot of his own book. (The magic of the wizard Merlin, who has an important role in this book, is mentioned as being derived from that of magical refugees who arrived in prehistoric Britain from the sunk Numinor/Atlantis).
The works of both writers include a set of beings who resemble the gods and goddesses of pagan pantheons, but who - though enormously powerful - are no gods, nor do they claim to be ones, but rather the servants or angels of a single true God, similar to that believed in by monotheistic religions and particularly Christianity (both writers were devout Christians). In Tolkien's works these beings are the Valar who serve Eru Ilúvatar; Lewis mentions the Eldils or Oyarsas who are subordinate to Maleldil.
And in the writings of both there is an evil rebel - Melkor/Morgoth in Tolkien's works, "The Bent One" in Lewis' - who is the source of Evil in the world, and seems related to the Lucifer of Christianity.
Further can be noted that the Mount Doom (chapter) - in which occurs the climatic final battle for the ring between Gollum and Frodo - has a close parallel in Lewis' "Perelandra". In both, a battle takes place in an underground chamber near a chasm at whose depth volcanic fire is burning; both battles end with "The Bad Guy" falling into the fire and being utterly consumed; and in both books, this outcome is vitally needed in order to deliver a whole world from evil domination.
- C. S. Lewis on Wikipedia