How do you get the marathon helmet in halo 3?

[Justin C]

I have all off line achievements and have all skulls, including the silver skulls. I unlocked the marathon man achievement but received no helmet, i have the security shoulders though, whats going on?

 

[quado]

to get the helmet you need all the achievements that includes online until then you wont get the helmet but when you get the helmet at the same time you get a katana and thats pretty cool also