The Game of the Few — John Taylor

The planet is inhabited by people living a medieval, pre-industrial existence in which there are magic, fairy-tale creatures and magicians. Arnulf the Wizard is driven by a series of dreams to gather a motley collection of others to go on a quest. At the same time, a starship is approaching the planet containing descendants of the planet's people whose own ancestors fled to another star system. Members of this ship's crew join with Arnulf's group to battle against the invading aliens. During their adventures, long-term, loving relationships are formed, life-long friendships come into being and a few tragedies occur. The saga ends with the promise that some of these adventures will go into space to carry the battle to the aliens. That is another story