He don't know the history.
This is an expression "have a history attached to it". It means that if something is old it been around long enough it gathered some events when it was a part of them. Just like "if only those wall could talk, I bet they could tell us some stories".
For the second part this is kind of plot explanation. The ghost have been around for so long it forget why it stick around. M tries to watch C. But after C death who he will be watching? And then he will forget. Unable to move forward or backward (as backward mean being alive again) it will be stuck in mortal plane.