There must be something in the air this morning, because all I can smell are achievements. The latest achievements to be confirmed are a fairly large set for Halo 3 bringing to light the previously rumored "Mythic" Map Pack. While it's unknown whether or these are single or multiplayer maps, we are going to lean towards Single Player and Co-Op Maps due to the existence of new skulls.
The original secret skulls were available one per map based on the level name, therefore the new skulls; Assembly, Citadel, Heretic, Longshore, Orbital, and Sandbox, should reveal the names of the new maps.
In total the Mythic Map Pack will add 500 Gamerscore to the recently added 250 Gamerscore for the title update that went live today. Looks like it's about time to fire up Halo 3 again and get the crew back together for some Covenant slaying co-op action!