To enter Black reach to get your hands on the elder scrolls you have to unlock this weird Locked Dwemer stair case that leads down to the Black reach door. I've found an exact same stair case in Miznchaleft when I went down there to get Mjoll's Grimsever. I tried to open the stair case but it said I was missing the necessary item. I'm sure it's the same cube like item you place on the stair case to reach black reach but I have no idea where it is or where to get it through starting a quest. Anyone able to shed some light on this, thanks for any help.