Gravity cannon
From Halopedia, the Halo wiki
The Gravity Cannon is a powerful energy weapon integrated into the Gravity Thrones of the Covenant High Prophets. [1]
Similar to the modified Assault Cannon wielded by the Hunters in Halo 2, it is seen only in the level Regret in Halo 2 and in Issue 3 of the Halo: Uprising comic. Only High Prophets can use this weapon. It is mounted on the front of the Prophet's Gravity Throne. There are two mounted in the front. Only one can fire at a time, though on Legendary it can fire both at a time. The beam is larger on Legendary, which can allow you to get closer due to the fact that the size will limit where the Prophet can aim it at.
The cannon is quite similar to the beam armaments used by Sentinels. The Covenant are known for their effective adaptation of Forerunner technology, so it is thought that the gravity cannon not only resembles the Sentinel beam but also based upon the beam's design. Its beam is a reddish-gold rather than the green of the Hunters' Fuel Rod Cannon and is more powerful than conventional Fuel Rod technology.
The Gravity Cannon is not seen in Halo 3. Its ammunition type is unknown but it is commonly believed to be a plasma-based weapon and thereby non-reloadable.
The Gravity Cannon is very similar in form and color to 343 Guilty Spark's energy beam, as shown in the levels Floodgate and Halo. The nature of Covenant weapons research suggests that the Cannon is hence a result of studies into Forerunner weaponry, but where they found the technology is unknown. They had no known contact with Sentinels or Monitors until the events of Halo, but they may have scavenged the technology elsewhere. It could also have come from studies into the Dreadnoughts' weaponary, because its arnament has been taken off of the dreadnought.
