The Oath: The Druid Chronicles, Book One

When the last of members of a secretive Druid cult are forced to abandon their hidden sanctuary, they send the youngest of their remaining priests in search of Annwr, their chief priestess's sister, who was abducted by a Saxon war band fifteen years ago. With only a rudimentary grasp of English and the ambiguous guidance of an oracle's prophecy, Caelym manages to find Annwr living in a hut on the grounds of a Christian convent.

Annwr has spent her years of captivity caring for the timid Aleswina, an orphaned Saxon princess who was consigned to the cloistered convent by her cousin, King Gilberth, after he assumed her father's throne. Just as Caelym and Annwr are about leave together, Aleswina learns that Gilberth, a tyrant known for his cruelty and vicious temper, means to take her out of the convent and marry her. Terrified, she flees with the two Druids--beginning a heart-pounding adventure that unfolds in ways none of them could have anticipated.