Cortana Letters
http://marathon.bungie.org/story/cortana.html#2 these included hints and teasers about the Halo series. I believe it contains clues about Halo 3 as well. Letters 1 and 2 seem more interesting to me.
http://marathon.bungie.org/story/cortana.html#2 these included hints and teasers about the Halo series. I believe it contains clues about Halo 3 as well. Letters 1 and 2 seem more interesting to me.