Probably not cause autosage. Throw a shoe at my head if I show up in another thread and you want to hear more.
But to give a baseline, Imagination is the stuff of dreams. Dreams are all about the creation/visualization of an idea.
The opposite of a dream is a nightmare. A nightmare is not the destruction of an idea, but rather the corruption of an idea.
So in rpg terms, shadow magic is debuffs out the ass.