Iron skull

The Iron skull is a skull available in Halo 2 and Halo 3. If active, when playing co-op, if either player dies the game restarts you at your last checkpoint. Additionally, if this skull is active in Halo 3, if the player dies playing solo the level restarts. 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. A 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 skull in Halo 3

 * Level: Sierra 117
 * Directions: This skull is at the end of the level. It is located behind the building where Sergeant Johnson is being held captive, up on the roof.  You can gain access to the roof on the far right side of the building.  Travel all the way over to the left side of the roof, and the skull is concealed in the corner.
 * Difficulty: Normal, Heroic, Legendary