Acho que isso é Enchantress from the Stars , de Sylvia Louise Engdahl. Publicado pela primeira vez em 1970.
"Elana belongs to a peaceful, technologically advanced, space-faring civilization called the "Federation", which monitors worlds which are still "maturing", allowing them to grow without any sort of contact or intervention. Elana stows away on a ship in order to accompany her father on a mission to a planet where intervention has been deemed necessary because a technologically advanced empire has invaded the planet in order to take advantage of its resources. In order to lead a young woodcutter (a native of that planet) against them (without exposing him to the truth about either alien civilization) Elana takes on the role of an enchantress. She gives him various tools, leading him to believe that they are magical." -Wikipedia summary
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"Elana poses as an enchantress and teaches Georyn and Terwyn, two woodcutter's sons, to use telepathy to scare the Imperials. Before this, Georyn and Terwyn are trusted with three tasks by the Starwatcher, an old man who is actually Elana's father. First, they must retrieve a magical disc (actually an inter-comm unit), a piece of the sun (an electric lamp), and a magic cup (an ordinary cup which Elana moves with psychokinesis) with Evrek, Elana's fiance, as the "demon" who challenges them to reach their full potential." - TV Tropes