Only clerics and priests without evil in their hearts could use this spell. On Toril, it was typically only used by priests of the Padhran religion, though it was not impossible for another deity to grant it to one of their own priests if they had specific need of it.
A cleric who used this spell was guaranteed that the target of the spell would never behave unethically toward them. Upon casting the spell, the caster determined the precise nature of ethics from which he would be guarded, namely theft, murder, truth, or breaking a promise. That is, a target placed under a "theft" karma curse could never steal from the caster, one under a "murder" curse could not slay the caster, one under a "truth" curse could not lie to them, and one under a "promise" curse could not break a promise they'd made to the caster.
If the target attempted to violate these conditions, the curse would be activated and they would rapidly lose experience and understanding, and indeed, karma, until they ceased. The target would be aware of the loss, but not necessarily the cause if they were not aware of the curse.
A karma curse could not be negated or removed by a remove curse spell. The number of potential targets for this spell rose with the cleric's skill. Once cast, a karma curse was permanent. The lost karma could only be regained by an equivalent act of contrition.
This spell only required verbal and somatic components to cast. Although the caster did not need to inform the target of the karma curse, they did have to tell them of the proper behavior expected of them. A caster could only make themselves the object of the spell, owing to its personal nature.
The massive golden statue of Mahavidi in the Room of a Hundred and Three Pillars in Saikhoi was protected only by a permanent karma curse. Anyone who touched it with thoughts of larceny fell victim to it.