Dark promise was a divine magic spell available to priests of Bane, the Black Lord. It put the victim under a type of geas that adversely affected his or her health each time the promise was broken. After Bane's supposed death in the Year of Shadows, 1358 DR, Cyric granted spells to Banite priests for a decade, so it was possible this spell could be found within the church of Cyric.
The caster had to touch the target of this spell as it was being cast. If the recipient was unwilling, they got one chance to resist the spell or be permanently bound to a dark promise. The promise had to be spoken by the caster, had to be an order that was in Bane's interest, and could not cause the victim to do something suicidal. Therefore, promises such as "Never eat", "Fight without weapons or armor", or "Don't think about purple orcs" would not work and the spell would fizzle. Appropriate commands included "Never tell anyone about this place", "Never attack a servant of Bane", and "For every ten silver pieces you receive in a tenday, give the church of Bane one silver piece".
Failure to keep the dark promise resulted in the victim permanently losing a bit of their health. Repeated willful violations of the promise could eventually result in death. This health loss could not be recovered by any means short of a wish spell or something equally powerful. This spell could be removed from the victim by the caster, by a wish, or by a remove curse cast by a good-aligned cleric of higher level than the caster.
Casting dark promise on a target that was already under the influence of a dark promise was guaranteed to fizzle.
Only verbal and somatic components were required to cast this spell. The caster had to touch the target, utter his or her name, and speak a carefully worded promise that met the criteria above.
This spell was primarily used in the early centuries of the church of Bane when his followers were hunted and persecuted. They used dark promise to help maintain secrecy and shift public opinion. As the church grew more powerful and widespread, use of this spell declined and it was rarely invoked.