The Wandering Fire (The Fionavar Tapestry, Book 2)

In Fionavar, Maidaladan, Midsummer's Eve, is approching but an unnatural winter is spreading all over the land. The Kings and Mages are gathering to try to understand the reason of this mysterious cold, and the armies of Brennin, Daniloth and Cathal are preparing for an oncoming war. Back in our world, Kim, now a Seer, summons Uther Pendragon in Stonehenge to help her wake his son Arthur in Glastonbury Tor, and crosses with the latter to Fionavar, for he is the legendary Warior who'll help them fight against Rakoth the Unraveller.