Halo is the term used to describe the Luxin that builds up in Drafter's eyes as they manipulate Luxin. In all cases except the Prism, drafting is an imperfect process, changing the body in ways that the body interprets as damage, which, among other effects, is the cause of the Luxin buildup.
When the Halo completely covers the iris of the eye and bleeds into the sclera, it is referred to as "breaking the halo." When this happens, drafters must either commit suicide or surrender themselves to the Chromeria to be executed, because not long after this event, the drafter starts to go mad. It is for this reason that drafters who are close to breaking the halo often undertake the annual Freeing before they actually break it. The details of the madness depend on what type of drafter they are: they begin to embody the traits of their Luxin (e.g. blues cold and distant, reds and sub-reds become creatures of rage, greens give in entirely to the wildness and ferocity of their Luxin, etc.) Many who do not die before the madness takes hold of them become Wights, who attempt to replace parts of their bodies with Luxin.