Overshadowing, primary

Because wizardry is a gift of the One via the Powers that Be, and because of the Lone Power's furious opposition to Their 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 unusual and unheard-of that some wizards are likely to say (as Nita does in AWAb) that it's impossible.)

However, this still leaves the unconscious route open. 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 when it's too late to stop it (in Pralaya's case, the situation involves a little of both). Overshadowing is routinely tragic for both the wizard 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 which are 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. TWD et seq.

(See also: Overshadowing, secondary.)