The Graviton Cannon is a super-weapon built by the Covenant Loyalists as a counter-weapon to the UNSC NOVA bomb. It is designed to be built quickly with a few components, despite its size. The pieces themselves are fairly large, and most of the cannon is pre-fabricated before being built on the planet.
Once constructed upon a planet, the Graviton Cannon can create a gravity well in a small area capable of dragging objects the size of a moon around in. It is very effective in against enemy fleets above planets, as it can move the ships around at will, causing them to collide.
As the Cannon is cheap, it is most effectively used to pull a planet's moon towards the planet. This will result in total destruction of the planet.