Halo 3 All Skulls Achievement - No Helmet

The other day me and my brother (ChiefANator117) decided that we would attempt to find all the skulls on Halo 3, and as we did we earned our Hayabusa chest and shoulder pads. As we entered the ark to gather the final skull, cowbells i think, we were shocked to discover that even with all the skulls no helmet was unlocked. So 343 what happened? I also noticed despite us both completing each campaign level on legendary, my service record says I have only completed the game on easy. I do not understand what happened, I can see the fault at this point is on your end, my service record and game both colaberate that I have all the campaign skulls, and that I have all the mythic map skulls, so I like to hear either form a member of the community or from somone at 343 themselfs to explain to me why the Helmet didn’t unlock and if thier is anyway to unlock it post collection.

You need to unlock every achievement in order to get the helmet, not just the ones for skulls.

We worked out the issue, on split screen we had all the skulls but individually we must have been missing one or two my brother was missing IWHBYD skull, I think mine must have been blind, though we played that level together at the end of the skull run so not sure about that one to be honest, I thing it was down to completing the level sierra 117 with the skull in hand, as the first time through we grabbed it, achievement unlocked and we ditched it, the second time and third time we both carried the skull through the level to the pelican. That’s may best guess as to what went down at least.