The Vampire Affair

The Vampire Affair is the fourth book of the Gaius Marius Chronicle, the memoire of a retired Roman soldier, Gaius Marius Insubrecus, a legionary who fought with Caesar throughout his Gallic campaigns and the Roman civil war, and who supported Caesar's heir, Octavius, against Caesar's murderers and finally against Antonius and the Egyptian queen, Cleopatra.
The Vampire Affair recounts Gai's first winter with the Roman army in Gaul. As the Gallic feast of Samon'win draws near, marking the end of the harvest and the beginning of winter, rumors reach the Roman commander from the dark forests of the north of bloody human sacrifice and a radical cult of druidical priests preaching the apocolyptic coming of a vengeful god who will scour the land of the Romans and all their supporters. As the Romans attempt to investigate these rumors, the Stryx, a blood-thirsty vampire from their worst nightmares seems to be attacking their soldiers.
Gai must travel into the dark forests of the north to unravel these mysteries before a bloody civil war ignites among the Gauls which will engulf the entire Roman army.