Bungie has just announced a new Halo game called Halo Reach. There is no word on what type of game it is, but it will most likely take place during the events of one of the first major plot points in the Halo timeline: the fall of Reach. Reach being a planet inhabited by Humans, that was taken over by the covenant. What makes the battle of Reach so important is because it is here that the original group of Spartan Warriors were wiped out, leaving Master Chief as one of the only Spartans in left existence. The game is set to be released in Fall of 2010 so it will most likely be a fully fledged Halo experience rather than an expansion like ODST.
