To claim the skull, you must have begun the campaign level from the beginning and have attempted it on a difficulty of at least Normal. The Sierra 117 campaign level can be accessed from the Campaign option on the main menu.;
, You are at the right section when you see a video clip of Johnson being kicked into a prison. There will be a dam there with multiple Covenant units guarding it, including one brute chieftain with a gravity hammer.
 If you feel like it, you can finish them all off; you will be able to collect the skull right before you finish the level. In any case, taking out the brute chieftain will help immensely, as he spawns in the area where the ledge to the skull is located.
 You will see a ledge on the right of the bridge, directly to the left of where the final pelican lands, with two wires leading to it - jump up onto it. It will have grating along it and run towards the far side of the large building on the opposite side from where you entered the area.
 This ledge runs directly left from the first ledge and winds its' way around the building, overlooking the holding cell of Sergeant Johnson and co.
 The skull is located in a little nook behind a pillar at the end of the building ledge, almost directly across from where you entered the area with the Arbiter.
 That is all there is to it. Check your Xbox 360 gamer score and achievement statistics to see the Iron achievement has been unlocked for 10 gamer score. It is represented by a skull with a hammer on it. You will also be able to use the Iron skull in your campaign games by pressing X in the campaign menu and selecting it. It gives you a x3 campaign score multiplier, but if you die during the level, you return to the start. If you are playing co-op, you only return to the last checkpoint.
Begin the Sierra 117 campaign level on Halo 3.
Play your way through the level until you reach the section where you must rescue Sergeant Johnson and his team from captivity.
Take out as many of the Covenant enemies as you can.
Move through the area until you reach the bridge over the dam.
Jump onto the blocked-off pipes at the end of the ledge and from there onto the building ledge above.
Follow the building ledge all the way until its end.
Claim the Iron Skull by pressing the Right Bunker button on your Xbox 360 controller.
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