Casting light? Very thematic! I've seen quite a few "only one light" games, but yours actually explains why there is only light, which I think is really cool. The first time the black mist appears, you guide the players attention to a crystal that has no function other than holding the light, which I thought was some clever explaining!
The movement is really clunky though. Being able to move in only one direction at a time, and needing to stop to move in another direction feels jarring, which takes away a bit of the magic for me.
Nonetheless, I think this is a cool concept, you should be proud!