Halo CEA Bandana Skull

There are skulls hidden around Halo CEA.  They unlock achievements, but they also modify the game in different ways.  They don't affect your achievements, so some of them can be very useful.  Arguably the most useful is the Bandana skull which gives you unlimited ammo, making those Legendary runs a lot easier.  Also negates the Famine skull to make it easier to get your 3 skull achievement.  Being the most useful, it happens to be the hardest skull to get (so far).  This video makes it a hell of a lot easier: