It has already come out but to get it you need to get HaloWars game to get it
the limited edition version has it.
27th of February 2009 with halo wars release
its going to come out with halo:ODST
they come out with the halo 3 O.D.S.T game
muythic map pack. that comes with HALO 3: ODST
its out now for 600 microsoft points
Mythic map pack mythic mature pack
if you buy the halo wars LCE you get a code for the new mythic map pack on Halo 3
A genuine code for a copy of the Horizon Map Pack for Halo 4 is available to you upon purchase of the map pack card from a store, such as Target, or you can purchase the map pack directly on Xbox Live via the Store on the Halo 4 main menu.
cant download halo 3 map pack, error message : premium is blocked
It will have a different campaign mode and all of the halo 3 maps but it will also come with both halfs of the Mythic Map Pack (Assembly, Orbital, Sandbox, Citadel, Heretic, Longshore).
Two multi-player map packs. Noble Map Pack and Defiant Map Pack