Gravity Cannon
From Halopedia, the Halo Wiki
The Gravity Cannon is similar to the modified Fuel Rod Cannon carried by the Hunters in Halo 2.
Seen only in the level Regret in Halo 2, 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.
It is a reddish-gold rather than the green of the Hunters' Fuel Rod Cannon and is much more powerful than conventional Fuel Rod technology. Some have speculated that the gun was originally going to be a Fuel Rod Cannon. This speculation arose due to a mod which showed the cannon's coloration to be green rather than reddish-gold. Some have also speculated that Regret was going to be a more formidable opponent as evidenced by this mod.
The Gravity Cannon is not seen in Halo 3. Ammunition type is unknown but it is commonly believed to be a plasma-based weapon and 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. This is seen as a reult of studies into Forerunner weaponry, but where they found the technology is unknown. They had no prior contact to the Sentinels or Monitors until the events of Halo, but they may have scavenged the technology off of a damaged or destroyed Sentinel, or after the Arbiter recovered the Spark off of the Threshold Mining Platform. It could also have come from studies into the Dreadnought, but that is unlikely since it has been de-weaponized.
