Soccer Ball (Forge Item)
From Halopedia, the Halo wiki
The Soccer Ball is an item for Halo 3's Forge gametype. It can be placed on any of the Heroic DLC Maps. It can be used as a rolling weapon with vehicle-like splattering capabilities, a shield since it is indestructible, or just a fun playing soccer ball. Game types and maps for soccer ball usage became downloadable content in the Bungie Favorites on Wednesday, December 12, 2007.
Using the Soccer Ball, some people play a form of Grifball. Instead of it being an Assault game variant, they use Forge and make soccer goals and try to whack them in with the Gravity Hammers, sometimes using overshields or changing game settings to keep players from dying. It is recommended using hammers or melee. It is an exact model of the soccer ball in Halo 2.
Using this soccer ball, you can also perform something which is called the Clone glitch. This glitch has no practical use just like the soccer ball, but can be fun to play around with.