Vidmaster Challenge: Annual

Vidmaster Challenge: Annual is a new achievement where you have to complete Halo after 9/25/08 on Legendary Co-Op with four players over Xbox live, Iron skull, and everyone in Ghosts .

It has been announced that while the Vidmaster achievements do not give you any points, they are worthwhile for some mysterious prize when you get all off them, although this cannot be accomplished until the Mythic Map Pack is released.

On the Halo 3 forums Sketch posted that you could get the Annual achievement before 9/25/08 because bungie made a last minute change to it.