Mental Magic

In "Witch of the Demon Seas" by AA Craig/Poul Anderson, Corun discovers that magic is not a gift of the gods or demons but two mental powers, geas and illusion. The evil sorcerer intends, in league with the amphibious reptilian race, to impose mental control on human populations in order to harness their innate mental powers, thus making himself and his allies even more powerful and even immortal. The interior of the reptiles' black castle, lit by luminous fungi, reads like a vivid nightmare.

Knowing that the sorcerer's apparent transformation into a much larger monster is in fact a hypnotically induced optical illusion, Corun continues striking with his sword until he has killed the sorcerer and thus ended the illusion. The sorcerer's evil daughter had been under his geas, thus not really evil, so she winds up as the story's heroine, not its villainess, after all.

In accordance with Anderson's realistic grasp of politics, Corun will enlist under Khoram the conqueror, thus winning a fairer deal for his own conquered people and helping to build a united peaceful kingdom. That is as much as I can say about "Witch of the Demon Seas."