I am playing the Eastern Pass scenario at the moment and control both the caves and the surface and thought it fun to try to take over the shadow world also, but no matter where I try I can't find a location where I actually can shadow shift.
Does anyone know a place on the map where you can? Is it even possible?

You are not meant to go to Shadow World at all. That map is only constructed that All-Devourer sends units from shadow world to attack you by one-way teleport.
There is no shadow world to speak of. There is only very tiny patch of land with Devoureres capitol na dteleporter. Nothing else.
If you really want to see it, it's in the left corner of map. Directly in it, you need to stand neer the corner or 2 tiles away at most.

Thx for the info.

Sad excuse for a Shadow World that is.