If you've finished the main quest or Dawnguard, then you probably have the attunement sphere stuck in your inventory too. The only purpose of it is to get to Blackreach, so theoretically it should be droppable after you gain entry....but it's not.

It's clearly a glitch and it needs to be fixed in the next patch. I'm one of those players that likes to have a clean inventory, so it's really frustrating having to hang on to this orb that has absolutely no use.