April 10 through 11, the multiplayer for Halo 3 will be free to celebrate the release of Bungie's third map pack called the Mythic Map Pack. Just like the other releases, this map pack, will have three maps bundled together in one package.
The maps included in the Mythic Map pack are:

Assembly


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. Some say it is similar to the two Halo 2 maps, Midship and Warlock. Assembly is fit for mid-sized Slayer and Team Slayer battles, and also supports objective gametypes, other gameplay details are indefinite. The map is symmetrical and only the second Covenant map designed for Halo 3 to date. If you look above, then a Covenant Scarab is being "assembled" directly above you, that is if you can stay alive long enough to see it being created all the way.

Orbital


Orbital is set on the Quito Space Tether, a UNSC space elevator. It is the first map in Halo 3 (Campaign or Multi-player) to be set in outer space. Orbital is described as having two main U-shaped corridors overlapping each other and having some interesting switches that grant access to certain areas of the map. The first public image of Orbital was released on Bungie.net in late December.

Sandbox


Sandbox is a tiered medium-large sized map, and 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 including, but not limited to, a golf club called the 7 wood, a Golf Ball, and a tin cup (golf hole). 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.