Overshadowing, primary

Because wizardry is a gift of the One via the Powers that Be, and because of Their opposition to the Lone Power's means and ends, only under the most bizarre and unusual circumstances can wizardry be used consciously by a wizard to do the Lone One's will. (Indeed, such occurrences are so extremely rare and unheard-of that some wizards are likely to say (as Nita does in AWAB) that it's impossible. But Haath in TBONWM presents such a borderline case, a single entity buried deep in an alternate reality where the Lone Power has spent a very long time perverting the nature of wizardry itself. Such circumstances fortunately require amounts of energy which even the Lone One is unlikely to want to expend on a regular basis...since Its introduction of entropy to the physical universe limits the amount of expendable energy for It as well as for all the other Powers.)

In almost all other cases, however, the stipulation leaves open the route through the unconscious. It is a rare species whose members do not experience some brief periods of unconsciousness of themselves or what they're doing: sometimes during sleep or similar states, but often enough when their attention is distracted while waking, or when their particular mental setup has left them available to be used by the Lone Power as a tool. This phenomenon is known as overshadowing, and allows the Lone One to force the wizard, puppet-like, into using his, her or its powers for evil.

The Lone Power's purposes are best served when the being in question never discovers what has been going on, or else discovers what's been happening only when it's too late to stop it and the damage has been done (in Pralaya's case, the situation involves a little of both). Overshadowing is routinely tragic for both the wizards in question and for their friends, who suddenly find themselves dealing with someone who seems to have turned into the worst kind of traitor, but may have no clear memory of what he or she has done, or even why.

Beings particularly susceptible to this kind of interference are those with multicameral minds, in which numerous personalities routinely handle different aspects of a being's life at once -- an organic form of multitasking, which to humans would look like some kind of multiple-personality disorder, but which in some species is perfectly normal. However, there are some multicameral species, like Sulamids, which seem immune to the effect. This may have to do with unusually high wizardly power levels.

There is some discussion among wizards who're interested in the theoretical or philosophical aspects of the Art as to why the Lone Power even bothers with overshadowing, considering how difficult the state is to establish, or maintain for very long. Some who have considered the situation think it's simply because, despite the difficulty, this is the way the Lone One can cause all involved -- the wizards, their victims, and the Powers that Be -- the most pain: the chief currency in which It willingly deals. (TWD, TBONWM et seq.)

(See also: Overshadowing, secondary.)