Like all mages Void mages do not have a choice in which magic they were born with. An unfortunate fact as the Void Mages are seen as especially unlucky and even monsters by most due to things beyond their control.
No one is sure what exactly causes a child to be born with a specific type of magic, and even less is known about the cause of Void magic. Despite this though, of all the types of magic Void mages are treated as if they chose the condition and are subject to the laws regarding 'their kind'.
Void Mages are incapable of interacting with other types of magic, they cause it to back fire, fail, or otherwise not work. Magical constructs near them are subject to critical failure and often self destruct violently if their magic touches them. In a magically reliant society this is a very very dangerous and bad thing.
There is no known way of blocking void magic from use or 'contaminating' an area. As such there is no treatment for the condition that has been found. Leading to some harsh realities for those affected.
Void Mages are especially rare with only one born every 50 or so years. Do to this fact not much is known about what causes a child to develop Void magic, there hasn't been much study done about it either due to the fear inherent in their existance.
While Void magic itself is not dangerous to the wielder, those who are discovered usually don't live past detection. If they do they are exiled and cast out of the flying cities to live among the Landbound, if they can convince those below to accept them. This however is not the common reaction as most determine it too dangerous to allow those who can destroy magic by merely existing to live among those who actively seek to destroy the city ships and those that live in the skies.