All the while you would be able to animate the illusion however you wished (walking, running, flying, burrowing, etc.). However, this maximum is only achieved when the illusion starts at the limit of the spell's range and is moved to the opposite limit of the spell's range. This does...
A creature that uses its action to examine the image can determine that it is an illusion with a successful Intelligence (Investigation) check against your spell save DC. This paragraph assumes you have made an illusion of a physical object. However your "dark fog" is not physical, things ...