How do i create fog of war on a minimap?

i have a minimap that uses a cartograph camera for a randomly generated grid-based dungeon. i even got a player icon to work by putting an icon on the players head and only using a layer the minimap camera can see. i want to create fog of war that clears on the minimap as the player progresses the dungeon. but between tutorials online and ai, i can’t seem to figure it out.