Mucius Furius Nero
Known as Nero the Usurper, this Patrician Senator of Thebes made a clandestine deal with outsiders from Terra Alia (specifically, Tasnicans) to acquire some fully-automatic weaponry (and the training to use it). After the disappearance of Clementia shortly after the Red Dawn offensive, Nero used the legion of troops he had trained and armed with advanced weaponry to seize control of Brundisium, Foundation (Fundamentum) and Thebes itself. Declaring himself "First Consul", Nero proceeded to eliminate all opposition to his rule, forcing Atticus and Caesar to flee the city.
Nero is almost universally regarded as a villain of the highest order in Thebes. If it had not been for his attempted coup, the Days of Wrath would likely have been a seige on the Empire with little lasting effect. Instead, because of the confusion and division that he wrought, invasions from Atalanis and Tawry went almost unopposed and two of Nero's three strongholds were eventually destroyed as he refused to surrender. The effects of his ambition will likely be felt for decades to come.
Nero was killed near the end of the Days of Wrath by a refugee Atalanean named Marcus Taridius, whom Nero had taken in (and proceeded to exploit) when he (Marcus) first arrived in Thebes.