Mythic Map Pack

Halo 3 has a new addition, the Mythic Map Pack. If you pre-ordered Halo Wars, you already have it, so skip this little announcement-like post. The Mythic Map Pack is another map pack for Halo 3. Each map pack consists of 3 new maps, and have their own playlist that requires the Download content. The Mythic Map Pack's maps are as follows: Assembly, Orbital, and Sandbox.
Assembly was originally planned to be in the Heroic Map Pack, then in the Legendary Map Pack, but was replaced due to the fear that it wouldn't be finished on time. The map is set in a Covenant Scarab Factory.
Orbital is set on the Quito Space Tether. It is the first map in Halo 3 to be set in outer space.
Sandbox is a crossover between the desert landscape of Sandtrap and the Forge flexibility of Foundry. Bungie stated that Sandbox will be "the ultimate Forge map". Many new Forge objects are added to give players more options to make their own custom-created maps. Bungie has also confirmed the tiered nature of the map. It is split into the "Sky Bubble", the ground, and the "Crypt." The Sky Bubble was designed so that Forge users could recreate maps akin to Guardian and Ascension since there is a grid only visible in Forge that will act as a death barrier, while the Crypt was added to create fully-enclosed arena maps using Grifball and similar gametypes. The ground was designed for vehicular maps more in the style of its larger brethren, Sandtrap. Every vehicle, weapon, and piece of equipment in Forge can be placed in the map.
The Mythic Map Pack is selling for 800 Microsoft Points on Xbox Live now.

Some info from Halopeedia.