Pendragon

AD 367. In a frozen forest beyond Hadrian's Wall, six scouts of the Roman army are found murdered. For Lucanus, known as the Wolf and leader of elite unit called the Arcani, this chilling ritual killing is a sign of a greater threat.
But to the Wolf the far north is a foreign land, a place where daemons and witches and the old gods live on. Only when the child of a friend is snatched will he venture alone into this treacherous world - a territory ruled over by a barbarian horde - in order to bring the boy back home. What he finds there beyond the wall will echo down the years.

Book 1 in the Dark Age series.