Gravity Throne
From Halopedia, the Halo Wiki
Gravity Thrones are the mode of transportation for the higher Prophets of the Covenant. Instead of walking, they choose to sit on these levitating thrones most of the time. The High Prophets have a version which is equipped with a shield that is impervious to nearly all small arms fire.
It is also equipped with a modified version of the Fuel Rod Gun. This weapon is called a Gravity Cannon.
Lesser Prophets do not appear to use these thrones as often, but Prophets serving military roles (such as the Legate) have Gravity Thrones of their own, though the version they use is less advanced. Other, usually lesser, prophets also use Antigrav Belts.
The High Prophets move about on gravity thrones rather than walk. The throne is equipped with a shield that is nearly impervious to all projectile weaponry, and is armed with a powerful energy cannon for defense.
Fleet Master Elites use a lesser but similar Gravity Throne without weaponry as seen in Halo 3. Ship Master Rtas 'Vadum sits in one and gives it to the Arbiter in the end.
In Halo 2, various mods made the Gravity Throne a drivable vehicle, though this variant did not feature a usable Gravity Cannon though more mods made sure to include the weapon.
