Halo 4 will have community-created mod Grifball as one of its main multiplayer modes, 343 announced at a panel at PAX Prime moments ago.
The mode operates similar to footy where players try to take a ball to their team's designated goal. Players can throw the ball like a grenade and other players can catch it.
The two weapons that make up this mode are the hammer and sword. The hammer grants an instant kill and has a lot of range, but when you kill the ball-bearer, the grifball will go flying. The sword meanwhile is more unwieldy, but kill someone with the grifball and it automatically gets sucked into your possession.
The throwing and catching mechanics have been applied to the series' keep-away-esque Oddball mode as well.
Elsewhere, Capture the Flag has altered the role of the flag-bearer a bit. Now, they wield the flagpole in one hand and a magnum or "flagnum" in the other. The flag itself can also be used for slick melee kills where players use the pole to impale their opponents.