Marching With Caesar-Catualda the Usurper

When Gnaeus Pullus was released from the Praetorian Guard to return to the 1st Legion, he was certain that his days of dealing in the political intrigue created by his social superiors was over. He learns differently even before he and the 1st return to their home base in Ubiorum, when a random event occurs in the form of a man of his new command committing murder in a taverna in Mogontiacum. As The Fates decree, the victim is a member of the 5th Alaudae, a Legion with which Pullus has his own history, and in this incident the Primus Pilus of the Legion, Quintus Nerva, sees an opportunity for vengeance against Pullus for his savage beating and the subsequent suicide of one of his Centurions.
Nerva's desire for revenge happens to coincide with a larger plot to subvert a plan, originally concocted by Tiberius' natural son Drusus Julius Caesar, to replace the longtime King of the Marcomanni Maroboduus, with an usurper by the name of Catualda, a nobleman of the neighboring Gotones tribe. The subversion of Drusus' idea is facilitated by the acting Quaestor Gaius Visellius Varro, beginning with a false confession extracted under torture by Varro's henchman Canus from Pullus' Legionary, that Pullus was aware that his man intended to commit murder, giving Varro the leverage he needs to force Pullus to participate in Varro's subversion of Drusus' plan.
This version ostensibly calls for Pullus, under the watchful eye of Canus, to escort a Gotones nobleman hundreds of miles across Germania to meet with Catualda and inform him that Rome has agreed to place Catualda on the Marcomanni throne, but as Pullus and his scribe Alex, who refuses to be left behind, quickly learn, nothing about this mission is as it seems. Not only is that "nobleman" Catualda himself, but Roman assistance comes in the form of a full Roman Legion, none other than Nerva's Alaudae, who have crossed the Rhenus and are waiting for Pullus and his party to join them.
Together, but in an atmosphere of mutual suspicion and distrust, this small force must cross hundreds of miles, through the lands of tribes that had recently been implacable foes of Rome, to reach the Marcomanni capital of Casurgis, a hilltop citadel just south of modern-day Prague, where a weakened but still dangerous foe in Maroboduus awaits. Over the course of the ensuing events, Pullus learns that nothing is as it seems, that the reach of Rome extends hundreds of miles beyond the Rhenus, that men he considered enemies are not, and that Rome is making a mistake in their choice of Catualda the Usurper as the new King of the Marcomanni.