Halo 3: Mythic is the Halo 3 multiplayer disc that comes with Halo 3: ODST.[1]
The disc includes Halo 3's multiplayer component, Forge, the Theater, and every Halo 3 multiplayer map, including all the released DLC the 3 new mythic maps, Heretic, Longshore, and Citadel.
Also sporting new background images and music.