Last night, Bungie revealed the second part of its original Halo 3 trailer (prepare to drop) as the trailer for the new Halo 3 campaign addon, Recon (Recon is whole new game using the H3 engine). Halo 3: Recon tells the story of what happens on Earth before Master Chief falls to Earth at the beginning of Halo 3. In the expansion, due in Fall 2009, you play as an ODST (Orbital Drop Shock Trooper) holding down Africa waiting for Master Chief to return. The expansion is supposed to be tougher (as you do not have any shields), and so you will have to employ more stealth than before.
According to Microsoft, Recon will ship with new multiplayer maps (that are different from the Mythic Map Pack) and will improve on Halo 3’s Forge Engine (yay).
You can watch the full trailer (includes new content) here. The game will be availible in stores on a disk (not on marketplace currently). It looks mighty pricey.