First go to Quarintine Zone. Next look around to your right side and you should see a small tunnel. Walk through it. Now keep on going strait till you see a strip of land that is connected to the wall (There are many strips of land connected to the wall so pick the one that has kind of like a part of a ship or metal hiding it). Walk down the strip of land ( careful not to fall off ) till you reach the end and you should see a floating skull. You must do this on Legendary.

This skull makes things have bigger explosions and more
powerful melee attacks.