Tyson "Ferrex" Green has written up the first part of a three-part series on working in Forge, Halo 3's in-game editor. You'll apparently be able to call on a startling variety of objects in the editor; although you won't be able to change map geometry, you'll be able to add objects so large that they will, effectively, change the flow of the map.

And, of course, it's not just editing, it's also a play mode:

Unlike that most famous of Monitors, 343 Guilty Spark, you’re still vulnerable to damage, but it takes a steady shot to hit a moving Monitor. Evasive maneuvers will serve you as well as a gun, but there’s a bit more to Editor Mode than simply dodging bullets. You see, Editor Mode is where the magic happens.

Check out the first part and be sure to watch for the rest coming soon!

Thanks for the heads-up to JawReich in #hbo.