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] Ensemble has confirmed that the unit has been cut[2] from the final game.
It may have been replaced, in the game by the new Vampire.
References
File:Covenant Air Artillery Concept.jpg
Covenant Air Artillery concept art.