story identification - Some aliens help a struggling planet

I read a book a few years ago from the 1980s, probably about a group of people from a planet going onto another planet undercover to help save them. The planet they went to was based in something resembling medieval times. A few trainers — one was named something along the lines of Gerard — trained a few people on the planet in telekinesis. The opening scene was actually an epilogue. I remember the trainers assigning challenges to these people; one was to go into some dark forest overnight. It was pretty good and I am interested in reading it again. At the end, the trainees had to fight some giant machines of another planet that wanted to take over. The cover was dark blue and I have no clue who wrote it. Thanks in advance.

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