The Covenant Air Artillery was an early airborne Covenant weapons platform armed with three Plasma Mortars (also the main armament of the Wraith). As it had no on board defenses, it could have been very vulnerable if dealt with promptly. However, its heavy plasma mortar cannons could lay waste to an entire UNSC base.[1] This unit has been cut[2] from the final game.
It may have been replaced in the game by the new Vampire because Ensemble Studios thought that The Covenant already had too many ground units. It was also the first unit to be deleted from Halo Wars.
References
File:Covenant Air Artillery Concept.jpg
Covenant Air Artillery concept art.