Watch this. Than watch the ign video with luke from bungie that he links.

Thanks, but it seems like I'm not going to know much about nightfall—other than that it's hard and level 20 is required—until the game comes out and I play it. I was hoping that it was a game mode that required you to fight for the survival of the Last City. Because bad guys do that at night, right? They attack your stuff when it's dark out. Everyone knows that. It's a psychological thing.