Alpha 2D shaders

Hi guys !

I'm a total noob when it comes to shaders so maybe this is actually super easy but, i'm making a 2D side scroller and i'm looking to have pixel circles around some object in the scene act as masks and display full brightness within their range, while a darker alpha outside of them. It's the exact same effect as in Samurai Lantern in the video here:

How would you go about implementing that?

Cheers !

M