scenic torrent
I suggest you start with this page: https://docs.godotengine.org/en/stable/tutorials/2d/2d_lights_and_shadows.html
While there is the option to have light set to subtract, which is kinda like negative light... I don't think this is what you need for this.
You know what most people call "subtracting dark"? Light.
It's not negative darkness, it's called light.
What you want is a simple light source on the player, while the rest of the world is dark (lack of light).
hallow birch