Crossover fantasy series by Guy Gavriel Kay made up of three books -- The Summer Tree, The Wandering Fire, and the Darkest Road.
5 students from Toronto are transported to Fionavar, 'the first of all the worlds', ostensibly to take part in a celebration for a jubilee.
Once there, they get caught up in a struggle between the powers of evil led by Rakoth Maugrim, the Unraveller, and the powers of light. Each of the five has a significant role to play in the war.
Kay takes elements from many of the world's mythologies, to make up this high fantasy trilogy, including Norse, Native American and Arthurian. Some critics denigrate this approach, intimating that there is just too much, but as it is a complete world, there is no reason why the mythologies of it should be any less diverse than our own.
Kay's prose style is beautiful and fluid, his characterisation detailed and believable, and his plotting compelling and engrossing. Be warned though -- in a Kay novel, bad things happen to good people. Some even die. And you will almost certainly cry when they do.
As an aside, 'Fionavar' or some variant of the name, is mentioned as part of the mythology of all the worlds in Kay's other novels.