Mirrors and reflections

(Maybe smut) (SunxMoon)

Sun Light has always had curiosity.
He never didn't believe in things.
In some cases his anxiety would be off the charts. In others his curiosity would replace the anxiety.
One day he finds a book about myths of mirrors.
He finds one saying that mirrors are actually windows between our world and another and our reflection is a creature or a demon on the other side disguised as you and trying to find its way out
It says if you turn the lights off, light a candle, and say your name backwards the creature or demon will show what it actually looks like.
Doesn't say that the window opens though.