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 into was based in almost midevil times. A few trainers, one was named something along the lines of Gerard, trained a few people on the planet telekenesis. The opening scene was actually an epilouge. I rember the trainers assigning challenges to these people and 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. tanks in advance