This article contains plot details from the latest book.
Shadowflux, a sixth element, is 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. On rare occasions, shadowflux allows itself to be contained, respecting another and seeing them as an equal. 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.
It is said to be an element, such as water or earth, as it dreams, hovering high above, waiting for someone to capture its interest. It is distant and has to be called for and convinced that it wants to respond, even then, is still a rebel. It is all things, waiting to discover how it will be wielded.
When encountered, shadowflux leaves behind an echo as a remnant. Echoes from shadowflux are a burden to bear and and will eventually fade in time. Shadowflux are struck with an emotion, and the emotion is what leaves behind an echo. The echoes haunt then person struck with pain, nightmares and affects the area struck. The area struck and with the echo will likely be affected, for example, if a heart were to be struck, the pulse and emotions would be affected, whereas for the head, certain thoughts would be affected. The specific emotion would change the toleration level and the time period it lasts. When your conscious isn't in control, the echo takes over, controlling your thoughts, memories and emotions. Though eventually fading, some tiny remnant will always remain.
In Flashback, Sophie was hit with spears of shadowflux, by the Neverseen member Umber, 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, resulting to them to slithering away instead of fading.
Later, throughout many lessons Tam gains control of this pure darkness, although he is often shown to be afraid of this strong power. It is revealed that he had let a bit under his skin, leaving an echo that was much calmer then Sophie or Fitz's.