Plasma mortar

Introduction
Almost exclusively outfitted on Wraiths, the Plasma mortar fires huge balls of slowly moving superheated light-blue plasma with a large explosion radius and slow rate of fire. It is extremely powerful and usually kills in a single hit and can destroy just about anything on the battlefield in a minimal amount of shots. It uses indirect fire, like the Fuel Rod Gun, meaning that it can be fired just about anywhere on a battlefield without having to worry about obstructions such as hills. The Covenant Air Artillery carries three of them for artillery bombardments.

Tactics
While in Wraiths, the Elites and Brutes that drive them usually use the plasma mortar at far distances. As the plasma is affected by gravity, meaning that it is shot at an arc, the driver always aims higher than the enemy, and always at long distances. In Halo 2 they are equipped with automated plasma cannons that only an AI can use.

If you boost the Wraith while firing, the arc will be less pronounced

Covenant Artillery
A giant stationary plasma mortar was seen destroying buildings in the Halo 2 E3 Demo video. Sergeant Banks called in an airstrike and a group of Longsword fighters flew in and destroyed it.