It is represented by a blue circle with the Iron skull in the middle. The skull has a hammer on its forehead. In the background are mini-skulls.
The Iron skull can be found on Sierra 117 on Normal, Heroic, or Legendary difficulty. To get it, at the end of the level at the dam, you must jump onto the pipe walkway and go to the farthest corner. It is easy to miss, so be sure to look around the crevice where it's hidden before giving up.
Effect: When either player dies in Co-op on any difficulty all players will restart at the last checkpoint. When a player dies in a single player mode on any difficulty, the entire level will restart.