The Queen of Air and Darkness by Poul Anderson (1971)
In this story, the planet Roland has been settled for many years, but large portions of its interior have never been mapped let alone visited. There have been persistent rumors of sightings of unusual, nonhuman lifeforms called the Outlings, but there is no objective evidence that these exist—at least not until Barbro Cullen’s young son is stolen from their camp while she is accompanying
a scientific expedition in a remote region. When the local authorities decline to help, she turns to Sherrinford, the only private investigator on the planet.
Sherrinford’s efforts convince him that there really are indigenous aliens 
hidden in the planet’s forests. Most human colonists outside the major population centers believe in them, though many refuse to acknowledge that fact. Although this provides hope that the missing boy is still alive, it also means that they are opposed by an intelligence that might be actively hostile if they get too close, an intelligence apparently augmented by telepathic abilities. The detective and the boy’s mother penetrate into an area avoided by human settlers, relying on modern technology to even the balance of power. There they are attacked by a variety of creatures apparently formed partly out of Earth legend, and the woman is briefly kidnapped by a figure she sees as her dead husband returned to life. Ultimately, reason overwhelms superstition and technology overwhelms the ability to project false visions. Sherrinford rescues the woman by proving to the other human captives that they have been taken in by an illusion. The indigenous aliens of Roland, who lived concealed in underground caverns, have stolen images and primal archetypes out of the minds of the settlers and used them to construct an illusory fairyland as part of their effort to subvert human culture and eventually destroy what they interpret as an invasion.
Although Sherrinford expresses hopes that the two species will learn mutual respect, we are left with a lingering doubt that the aliens can long survive contact with a technology based culture. Poul ANDERSON, who has written a fair amount of fantasy based on legends, explains that we cannot as a people escape the primitive archetypes of the past, and that it is only by understanding and acknowledging these cultural pressures that we can be free to adjust to new situations.
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