Barbarians from the Isle - Sigmund Brouwer

Winds of Light Book 2. New Lord of Magnus, Thomas must risk all in war against invading Scots. He soon learns survival is only one of his dilemmas -- barbarian rituals begin plaguing his army almost as soon as it marches north. Worse, his every move seems anticipated by masters of a dark conspiracy, and he soon doubts those closest to him. As a circle of evil closes around Thomas, he is given an ultimatum by the false sorcerers -- join their secret group, or lose his lordship and castle. Meanwhile, the God he has tried to ignore is patiently beckoning.