When you struck the man below the library with the moon touched rapier, you didn't see anything.
I don't mean he seemed the same. In the moment, it was like you didn't perceive anything at all. You followed through on your sword strike and returned to a defensive position, as you are accustomed to during fights. What you saw didn't even form a memory in your mind.
Later, though, you realize that something must have happened when the moon touched rapier made contact with his body. You sit and wrack your mind, trying to form the image of your encounter with him in your head. You pieced together a mental image of the scene. You can see the stone walls of the underground chamber, the various poses of your comrades frozen amidst their onslaught, your rapier outstretched and piercing the man's skin.
But you don’t see the man.
There is an empty space where the man should be. It isn't black, or white, or any color. It looks the same way the space behind your head looks. There's just a region of your memory where your mind assigns no meaning.
You sit in the library, eyes closed, and try to force the image into greater clarity. When you sleep at night, the scene occupies your dreams. You reassemble the details of the scene fragment by fragment, trying to cast a light on the place where the man should be.
But Marwyn Gandalc is too clever for her own good. So you come up with an alternative solution. You recall what you saw around the gap. You form a picture in your head that's so clear, you can tell where the scene ends and the nothing begins. You form, through excruciating recollection, the rough silhouette of the void that stood before you. You can't determine details, but you can sketch out the most basic of forms.
All you can put in place, after all that mental strain, is a roughly humanoid figure with a massive pair of wings.