The actual rarest ghost shell is a variant of the last city shell (damaged). For the new light players or the OG's that just forgot, when you started a new character in D2 you character started out with a basic white (common) last city shell. When Gaul yeets you off his ship and steals the light, your ghost gets "damaged" and looks all depressed. But what really happens is you are forced to equip a different ghost; a unique ghost you have for basically one mission. When you get your light your back your ghost is once again forced over to a new shell, and because the shells share the same name, when you go to pull said ghost out of your collection book their is only the post-light Last city shell (damaged). Because of this in order to keep the "depressed" shell you had to put on a different shell and put the depressed shell in your vault. However most players did not do this because...well, why would you, and as a result most players do not have that version of the shell and can no longer get it because as of new light the intro quest has been replaced. Thus you can't try and get it anymore, making it very rare indeed.