Forge

Forge is the Map Editor in Halo 3, confirmed in the EGM Scans. You will tweak elements in the map, such as removing crates or adding Fusion cores. You will also be able to tweak "minor" weapon (and possibly vehicle) statistics, such as "Ammo Capacity."

You take the role of a Monitor (Such as 343 Guilty Spark). You can then travel throughout the map placing, picking up, moving, and deleting objects on the map. You can also add objects as well, but you only have a certain amount before you can't add anything else.

In a sense this isn't a map editor that you would have expected. You're not actually building and designing maps, you're manipulating the existing maps to a certain way you would like.

Items that can be tampered with include crates and barricades, as well as weapon, equipment and vehicle spawns. The downside is that there is a "spending limit" meaning that after awhile you will no longer be allowed to spawn anything until you delete something. It has been stated that the UNSC Elephant will not be among the objects due to its size and specific purpose on maps. Throughout the time you're editing you can press up on the D-Pad and change to your multiplayer Spartan/Elite model and play.

Once you're done with the editing you can start a party over System Link or Xbox Live and see how it works. You can build crate towers, grab a vehicle underneath a teammate, or have a friend jump on a crate and move it throughout the map.

Forge will keep track of stats and kills. It has been confirmed by Bungie that you can save your creations. Also, if your creation is liked by Bungie, they will make it downloadable by other players for use.

It is also speculated that when you are a monitor you will be able to be destroyed. Mainly because in the August Issue of EGM, it shows a yellow spartan with a detachable turret shooting at a Monitor, and the reticule is red. Then again that is just speculation. Of course, the writers also made it sound as if you couldn't be killed as a Monitor. However, it also stated that you could "press up on the D-pad to become a Spartan and roll a grenade..." at a tower of fusion coils "and watch his work (and him) blow up", which still implies being able to be destroyed as a monitor, because it also mentions respawning as one.