The Shadowed Land by Signe Pike

Kingdom of Gododdin, AD 580: After defeating the Angles at the Battle of the Caledonian Wood, Languoreth, her daughter Angharad, brother Lailoken, and the warrior Artúr mac Aedan are reunited. But all too soon, fate pulls each back to their own path.

Artúr receives a mysterious summons from his father in Dalriada. Languoreth and Lailoken return to Strathclyde with the dangerous former bishop Mungo in tow, determined to maintain the fragile peace between the Christians and the people of the Old Way. Meanwhile, Angharad must travel deep into the shadowed land of the Picts, hoping to become the initiate of Briochan, a druid who practices the secret Celtic art of summoning weather.