This article contains plot details from the latest book.
Shadowflux are a certain type of shadows which most flashers prefer to pretend don't exist. Many choose to avoid the subject and think it's easier to focus on shadowvapor because elves understand the instinct. Shadowflux doesn't like to obey and is a tremendous fight to try and control. It is a very powerful shadow that can go through things such as force fields. The longer shadowflux mixes with your blood, the more it takes over. The results of losing something affected by it is from fifteen to thirty minutes. Under light, it appears murky and undetectable.
In Flashback, Sophie was hit with spears of shadowflux, which caused the shadows to sink into her skin, shredding muscle and nerve. It didn't listen to any of Tam's commands and he had to wrap shadowvapor around the shadows to control them, resulted them to slither away instead of fading.