The entire reason behind Viscount Kaecilius' existence was to oppose Riveria when the time was right. Being the leader of the first slave rebellion, his name was popular amongst the masses throughout the Kingdom.
All Sylvester had to do was spread the word before his armies started marching. The serfs would, on their own, come out and shower the procession of the army with flowers. With the previous rebellion, the slaves were made serfs. Now it was time for total freedom. Years had already passed, and the blunt effects of suddenly freeing all the serfs were going to be manageable.
So the Viscount's three-thousand-strong army prepared to march out of the city and make way to River City. Of course, they were against the odds, as Riveria had hundreds of thousands in its official army. However, Kaecilius had what King Conrad did not—friends in high places.