Who are the shadows that speak to Grand-Fisher?
It's never specified exactly who they are other than than the fact that they're hollows but based on the fact that they appeared to be more humanoid than most hollows it's safe to assume they were Vasto Lorde (strongest form of natural hollow). They also could be some of the Espada pre-arrancar since Harribel (#3) was revield to have been a Vasto Lorde it would make sense for other high level Espada to Vasto Lorde as well.