Iron (Skull)

If you meant the material, see Iron.

Iron is a skull found in Halo: Combat Evolved Anniversary, Halo 2, Halo 3, Halo: Reach and Halo 4. It has a similar effect in each of these games, and is a gold skull.

Halo: Combat Evolved Anniversary
This skull can be located in the very beginning of the level Pillar of Autumn. As soon as you are able to move freely in the Cryo room, walk to the back of the room towards the two large barrels to your left. The skull will be behind these barrels.

Halo 2
When you enter the second and final large Gravity Lift (do not confuse with the smaller ones throughout the level), 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.

Halo 3

 * 1) At the point where you must rescue Sergeant Johnson and his team, snipe as many enemies as you can and then jump down and cross the bridge.
 * 2) Keep to the right of the bridge until you see a blocked line with some pipes. Hop onto this block and continue.
 * 3) When you reach the end of this raised block, jump up into the balcony.
 * 4) Follow this balcony to the end. The skull is hidden behind a pillar jutting out from the building tucked into the corner.

Halo 2
In co-op play, death reverts both players to their last saved checkpoint.

Halo 3, Reach, CEA and 4
On top of its original effect in Halo 2, Iron has additional effects in all of its appearances in newer Halos. Firstly, a restriction has been added to solo too; death restarts the level for the player (reverting to the last checkpoint would be pointless since death already does this in single player). Also, the Revert to Last Saved Checkpoint option in the pause menu is removed so that players cannot cheat the mandatory level restart using it.

Trivia

 * The skull's absence in Halo Wars and Halo: Spartan Assault can be explained by the fact it would be redundant; death in both of those games already means restarting the level.
 * In Halo 2, the skull is partially useless, as on Legendary (the difficulty in which it must be collected) it's effect is already in effect, and its change is only noticeable if the player switches to another difficulty without switching off their console.