In this snippet by Tommy Hodgins we can see the effect where the entire page is behind a fog of war like effect and is barely visible. When you move the mouse around you can see that there is a spotlight like effect, that reveals the content in that area, it kinda feels like pointing a flashlight onto the web page.
If you are having trouble with the pen, try the archived copy on GitHub
Flashlight Mouse Pointer Snippet
See the Pen Mouse Cursor as Flashlight! ?? using CSS Variables by Tommy Hodgins (@tomhodgins) on CodePen.0