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no mate that skull your talking about is a proper skull it just appears on any difficulty but the effect only appears on legendary difficulty. trust me plenty of people have found that one...
Just go look at some faq site and see the list of skulls and how to get em. Then read about it and just find the one that matches your description :)
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The skull is called 'Assasins' and it makes all enemies have active camouflage.
This makes the game a lot tougher. Did anyone else think it was interesting how their eyes stayed visible?