Essential to the genre of fantasy is the battle between good and evil. This does not mean that fantasy is simplistic in its morality. As Lewis noted in his review of Lord of the Rings, the characters, even Gollum, cannot be reduced to being either wholly good or evil even as good and evil are real forces that people must choose between. The challenge lies precisely in the fact that the characters are mixtures of these forces and the fight is less about defeating Sauron and his orcs, but the evil within.
Tolkien's surprisingly nuanced understanding of evil is rooted in Tolkien's Augustinian worldview where evil is not an independent power, but a corruption of the good. There are two implications of this. One, the Devil was created good by God only to fall. Two, the Devil is fundamentally uncreative. He can take those things created good by God and corrupt them. Similarly, Sauron was created good before being seduced by Morgoth. Even Sauron's ability to create the rings of power was rooted in that element of good within him. Otherwise, he never would have been able to deceive the elves, who, contrary to the Rings of Power show, could never be accused of being fools.
This fact that Sauron is fallen good rather than purely evil, creates the fundamental threat of Lord of the Rings. Worse than Sauron taking over Middle Earth is the possibility that someone (whether Frodo, Aragorn, Boromir or even Gandalf) will take up the ring to fight Sauron and become dark lords themselves. If Sauron was simply evil and not fallen, we would never take this threat seriously. To reinforce this danger, we have the character of Saruman the White, who was good for millennia only to fall in the years leading up to the story. If Saruman, Gandalf's superior on the White Council, could fall than even Gandalf is not safe from the ring's corruption. Ultimately, it is not Sauron who is the primary villain, but the ring and, by extension, the very people who are tempted to use it.
Evil's importance to fantasy is inseparably tied to magic. To accept the existence of magic means to see it through one of two conflicting perspectives. To be evil means to see magic as power that some people can wield to place themselves above others, not just physically, but also morally. To be good means to see that magic indicates the existence of a higher power who created magic and to whom even the magic user must submit to. In the best of fantasy, while the dark lord may be out there to serve as the catalyst for the plot, the real struggle will be within the hero. Characters will fall or be redeemed based on what magic ends up meaning to them.
An example of the evil perspective can be seen in Glaucon's argument about the Ring of Gyges in Plato's Republic. In Glaucon's telling, morality is something that weak people invent. Anyone with power, say a magic ring that could turn them invisible, would quickly cast off all moral restraints and commit adultery and even murder. In the real world it is easy to see how political power can render people empathetically tone deaf. It is not hard to imagine a world with even greater power divides, such as between those with magic and those without, leading to magic users seeing non-magic users as animals to be killed for sport.
This view can also be seen in the character of Uncle Andrew in Lewis' Magician's Nephew. Andrew tricks Polly into teleporting into another world and then blackmails Digory into going after her to rescue her. In his defense, he declares:
I am the great scholar, the magician, the adept, who is doing the experiment. Of course I need subject to do it on. Bless my soul, you'll be telling me next that I ought to have asked the guinea-pigs' permission before I used them! (pg. 23)
Digory has the same facts as his uncle, but sees the moral truth beyond them. He, therefore, responds:
I didn't' believe in Magic till to-day. I see now it's real. Well if it is, I suppose all the old fairy tales are more or less true. And you're simply a wicked, cruel magician like the ones in the stories. Well, I've never read a story in which people of that sort weren't paid out in the end, and I bet you will be. And serve you right. (pg. 24)
The existence of magic baptizes Digory's imagination so that he can no longer accept materialism, but, unlike his uncle, what he sees is not power that he can use to place himself over others. Instead, he comes to know a moral order that is as real as any physical object.
While Uncle Andrew is a dilatant magician, playing around with things that he does not understand, Digory soon find himself dealing with the far more dangerous Queen Jadis, who destroyed her entire world. She feels no guilt about this. but declares:. "I was the queen. They were all my people. What else were they there for but to do my will." (pg. 61)
To be clear, Jadis, at this point, lacks the malice to be truly irredeemably evil. Instead, she comes across as more of a spoiled child. One imagines that she could still be saved if she could only have some sense smacked into her. The point of no return for Jadis is when she eats the apple and attempts to convince Digory to do the same. She is motivated by the desire to gain an even greater level of power for herself, mainly immortality. Digory is tempted by his desire to save his dying mother. What holds Digory back is that, by this point, he has come to know not just an abstract moral law, but the person of Aslan, who has commanded him to not eat the apple. Digory chooses to remain a normal boy, who will grow old and die but still have a relationship with Aslan. Jadis, seeing only power, chooses to become the White Witch. She may be immortal and destined to rule Narnia for a hundred years, making it forever winter and never Christmas, but she is forever beyond redemption.
