DepthMask shader that alters transparency

I wish to create some sort of mask that will render meshes behind it more transparent.
At the moment I have a Z-buffer shader that completely hides objects behind it. I want to add a transparency effect, preferably with a property where I can set the transparency value. Since I am very noobish in shaders I have no clue how to do that

This is how my example looks so far. The white border represents the edges of the Quad with the mask.




Thank you for any help!