The Pendragon's Quest. The Last Pendragon Saga, #4

Although the dark forces of Arawn are defeated, the Saxons have massed on the border of Wales, aiming for nothing less than the complete subjugation of the Welsh people. But defeating an army of Saxons is only the first step Rhiann, Cade, and their companions must take if they are to stop Mabon from claiming dominion over the human world for all eternity. The Pendragon's Quest is the fourth book in The Last Pendragon Saga.
The Complete Series reading order: The Last Pendragon, The Pendragon's Blade, Song of the Pendragon, The Pendragon's Quest, The Pendragon's Champions, Rise of the Pendragon, The Pendragon's Challenge, Legend of the Pendragon.