I used to cast Soul PLague when I had a Greck Egg on the field. The egg hatched but after, I was unable to move the Greck! Some hexes around it are darked, and it was unable to be dimmed with Goblin Shawmill.
The most strange thing: I moved a Faceless Lord in the darked area and it was unable to move further! And sure, it got the same "Space isn't revealed" when I moved cursor over it for the Shawmill.

Look at the screenshot> Space isn't revealed