Arcane Legions: Siege Engines of Rome

From the box:

The Romans spent centuries perfecting the science of war, and the Night of Mists only provided them with newer, deadlier ways to control the battlefield. Now you can control Rome's latest technological advance the Steam Ballista. Designed with the help of reclusive dvergar engineers, this massive artillery piece uses magic to heat water and channel steam to propel explosive stones or bolts the size of tree trunks into the enemy ranks with deadly precision.

These lumbering weapons are fielded one to a formation base but are operated by everyday soldiers from your army packs choose between two different unit cards for each ballista to find just the right way to show the enemies of Rome what it means to face the wrath of the Empire.