i can’t get the skull on Heretic everytime the lever tries to load it just takes me back to the main menu, it happens on the pc and xbox 
isnt heretic just a cutscene only level?
I’ve never heard of a skull on the cutscene only mission The Heretic. Unless you mean the Heretic skull on the multiplayer map in Halo 3? If so I don’t know why it’s not loading.
Have you gotten it in mp at all? If you have I’m really not sure but if you haven’t the file for the map could be corrupted if you can’t load the map at all. If you use mods something might have gotten screwed up as well. Worst case scenario you could just uninstall Halo 3 and install it and see if that works. It’d be better than uninstalling the whole collection.