I’ve got the first 5 skulls. I haven’t played further into the campaign yet because I want to restart on legendary.
They only do their thing if you turn on the meta-game option, which I guess gives you a score on your campaign missions.
I want to get all of them for the hayabusa helmet…then 1000/1000 gamerpoints gives you the samurai sword on your back…pimp all the hoes 