Merlin and the Land of Mists Book Four: the Druids

Camelot is in deep trouble, for not only has Merlin's father, the Elder god Mithras Invictus, left Avalon possibly never to return, but King Uther Pendragon has ordered that all the Elder god's shrines and altars are to be pulled down. This is not a great idea when the Lords of Winter are already at the Gates of Avalon and are waiting to bring the Cold of Eternal Winter to Camelot and its surrounding lands. The Druids are desperately trying to defend Avalon but they can only hold the Lords of Winter at bay for a short time. The young Druid girl, Ceinwen, comes to Camelot in an unsuccessful attempt to stop the destruction of the Elder god's altars. Once again it is left to Merlin, Galahad, Kraak and Grim to stand and fight for the survival of Camelot and Avalon.