Clash of Legions — Anthony Riches

Marcus Aquila and his patron Rutlius Scaurus are tasked with rooting out a spy ring operating in the lands into which their imperial master Severus's armies are advancing.
The quest to find and subvert their foes' informers will place the two friends at great risk, with torture and death the price of any mistake. While success will put them in the front rank against battle-hardened legions, hungry for revenge, in a bloody struggle to determine the fate of Asia's loyalty.
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'This is fast-paced and gripping "read-through-the-night" fiction, with marvellous characters and occasional moments of dark humour. Some authors are better historians than they are storytellers. Anthony Riches is brilliant at both.' Conn Iggulden on Empire
'A master of the genre' The Times