From audible.com:

The Summer Tree is the first novel of Guy Gavriel Kay's critically acclaimed fantasy trilogy, The Fionavar Tapestry. Five university students embark on a journey of self-discovery when they enter a realm of wizards and warriors, gods and mythical creatures - and good and evil....
It all began with a lecture that introduced five university students to a man who would change their lives, a wizard who would take them from Earth to the heart of the first of all worlds - Fionavar. And take them Loren Silvercloak did, for his need - the need of Fionavar and all the worlds - was great indeed.

And in a marvelous land of men and dwarves, of wizards and gods, five young people discovered who they were truly meant to be. For they are a long-awaited part of the pattern known as the Fionavar Tapestry, and only if they accepted their destiny would the armies of the Light stand any chance of surviving the wrath the Unraveller and his minions of darkness intend to unleash upon the world....