I just paid two hears for Spirit of the Night (whose lower-right hand message was "Scary").
Did I just pay two hearts so that I could fly? What does "Scary" mean, and what does Spirit of the Night do?
In addition to the flight ability that you observed, Spirit of the Night also makes your shots spectral (i.e. they pass through anything but enemies and rarely interact with environment items that you can shoot, such as fire and poop). As far as I can tell, this is identical to the effect of the Dead Dove item.