A Vanisher is an elf with the special ability to allow light to pass through their bodies, causing them to become invisible. When a Vanisher is enhanced, the ability becomes automatic - light will automatically passes through them. The length and quality of the vanish is dependent on how often the Vanisher trains.
The effect has no affect on gnomes, however, who can still see them due to the pollen and dust that settles on Vanishers. Biana and Alvar, however, have discovered ways to hide themselves from the gnomes. Vanishers sometimes don't even realize they blink in and out of sight as they move. Alvar used his vanishing ability to kidnap people (including Sophie and Wylie) for the Neverseen. Talented Vanishers, such as Wraith, can partially vanish, as when he is with the Black Swan he is only a floating cloak. Vanishers also seem to be able to flash-step; intermediate and advanced Vanishers can do so. This was shown when Della appeared in the water without getting wet, and Biana vanished to the other side of the room. When Sophie enhances Biana she can turn three people invisible at a time (including herself).
“She blinked out of sight, reappearing a second later knee-deep in the river. Sophie wasn't sure what was crazier, how fast Della had moved, or how she hadn't caused ripples in the water.”