“She couldn't move her head. She couldn't feel her body. It was like her brain wasn't connected anymore, and she had an overwhelming urge to let go, drift with the gentle breeze tugging at her skin and follow after the parts of her that the light had dragged away.”
—Narration, in Keeper of the Lost Cities
Fading away is when someone loses too much of themselves in a leap. Light leaping requires one's molecules to separate in order to "hitch a ride on a beam of light," so fading occurs when an elf's body isn't able to fully reform, and eventually, the light pulls the rest of them away and their particles are scattered forever. It has only happened a few times in history. According to Elwin, a mild case is when the color is washed away from an elf's body. A more serious case is when their skin starts to turn translucent, and it is deadly when they are practically clear. The main cause of it is lack of concentration. Elves wear Nexuses until their concentration is enough to prevent that. Murder by light leap, however, is possible. Causing someone great pain or shining another light into the path they're following are the two known ways. Gethen (under Fintan's orders) murdered Cyrah Endal by distracting her and causing her to lose her concentration while light leaping.
Sophie nearly faded away in the first book because she wrapped too much of her concentration around Dex while escaping their kidnappers. Elwin had to make her a special limbium-free Fade Fuel to save her life. Her recovery took two weeks. Later, in Exile, she felt dizzy whenever she leaped and continued to fade slightly when leaping. It was later discovered that the light had created a breach in her impenetrable mind that was only fixable by drinking an ounce of limbium, which she did near the end of Exile. Drinking Limbium also served the purpose of fixing her abilities.