The Heroic Map Pack includes three new maps, Standoff, Rat's Nest, and Foundry. It was released at 2:03 AM on December 11, 2007. Available for 800 Microsoft Points exclusively on Xbox live Marketplace, each map presents a totally different play style. There are a whole range of new Forge items to be usable on these maps that were just a static part of the environment on previous maps, like shield walls and mancannons, and other objects, like a big soccer ball in Foundry. It has been announced that the Heroic Map Pack will be free[1] later in the Spring of 2008.
Maps
Standoff
The symmetrical valley of Standoff, with its entrenched bases and fields of boulders is ideal for mid-sized objective and Slayer game types. There is a secret forge item for this map.
Rat's Nest
The vast, labyrinthine passages of the Rat's Nest bring something completely new to the multiplayer experience: an indoor vehicle paradise. Strongly influenced by the Campaign level Crow's Nest, this map is ideal for big team battles. You can have a scorpion indoors on this level.
Foundry
Foundry is the ultimate Forge map. Players can edit every single object in this voluminous industrial warehouse, placing stairways, walls, bridges, and tunnels to create an entirely new play space and build almost any kind of map imaginable.
Extras
In all the maps listed, there is a secret forge item, a large football, rather like the one hidden in Metropolis on Halo 2.
Sources
http://www.xbox.com/en-US/games/h/halo3/news/20071119-heroicmappack.htm