Before you read below, I though that since the disc comes with ODST, it'll be fine here. Feel free to move this if you feel I've chosen my sections unwisely. Also, if these questions have been answered in a blantently obvious way (such as Bungie Weekly Updates), don't troll, just answe the question. Moving on... 1. First off, are the Halo Mythic discs and Halo 3 discs compatable? It seems like having Halo 3 achievments that you can only get on a different game seems odd. If yes, move on to question number 2. Don't answer the next question if they are uncompatible. 2. Let's say you are playing Halo 3 Mythic, and another person is playing Halo 3. You are both in an XBox Live party together, and the other person switches over to the Campaign lobby. What happens to you, the person who doesn't have Campaign on the disc?