Iron skull

The Iron skull is a skull available in Halo 2 and Halo 3. When the skull is active in co-op, it sends all players back to the last checkpoint on the occasion that one player dies. This is effective in both Halo 2 and Halo 3. When active in a solo campaign mission in Halo 3, the player is restarted at the beginning of the level in the event of that player's death. In Halo 3, the revert to last checkpoint option in the menu doesn't even appear, since Bungie has removed it. In Halo 3, this skull provides a 3x point multiplier if activated during the meta-game. Finding this skull in Halo 3 awards a player with the Iron Achievement.

Finding the skull in Halo 2

 * Level: High Charity
 * Directions: When you enter the final (the second) Gravity Lift, look up. The skull will pass through you extremely quickly so you must start holding X a bit before you actually pass it to grab hold of it.
 * Difficulty: Legendary

Finding the Iron skull in Halo 3

 * Level: Sierra 117
 * Directions: This skull is at the end of the level.  It is located on the roof of the building behind the one where Sergeant Johnson is being held captive.  You can gain access to the roof on the far right side of the building. Simply jump on the grate between the middle level and the water. Then jump up to the roof as high as you can. You may jump on the pipes and then to the roof.  Travel all the way to the opposite end of the roof and then look for the skull concealed in the corner on the left side.  Note: You must start at the very beginning of the level, otherwise the skull will not appear.
 * Difficulty: Normal, Heroic, Legendary(The difficulties are the same for all skulls in Halo 3)