From 1529 to 1538, or the Duplicity Period, those of evil nature and/or criminal intent were greatly helped in deceiving their victims. A powerful spell masked certain divination magic and as a result people were cheated, lied too, and a few noble creatures fell from grace (c.f. Manjular).
In 1529, Neld-Rac invoked a powerful spell on all mortals of Bal-Kriav. This spell drew on the energy emanating from the Chaos Malestrom. This was needed because even he has a Higher Power did not have the power to evoke this ancient spell. When the spell finished, for ten years on the world Bal-Kriav, detect alignments, magical disguises, and similar divination magic usually failed or produced incorrect results.
The Balance was furious with Neld-Rac's violation of the Mandate of the Heavens, rules they must all adhere to if they are not to fall victim to Chaos and its primordials. They were also worried because Neld-Rac used a forbidden spell, one thought destroyed and forgotten, called the Wind of Duplicity. Long ago, this spell, created by one of Asmodeus's lieutenants, was used to mask their doings on Ellubôz; doings that would lead to them becoming Fallen Angels (9414 GE).
It took the Balance and their minions almost a decade to find a means of reversing the spell. They punished Neld-Rac by reducing him to a minor power. This punishment lasted 250 years and caused Neld-Rac to look for a way to get revenge on either The Balance or their minions.