How to cull an object that goes behind another object, with invisible mesh

I’m doing at.
I want an object with a mesh [a], to move behind another object, which is invisible , and get culled as if was not invisible.
I want to make my invisible object, an object I can put in place of a real wall, from the cameras image; but I don’t want it to be actually visible.
That way I can make ar objects go behind windows, walls, doors and look nicer.
I have no idea how to achieve this. Help gladly received.

Thanks I found it http://wiki.unity3d.com/index.php?title=DepthMask