Candle of truth

This white tallow candle, when burned, calls into place a 20-foot radius zone wherein all creatures are affected as if by a potion of truth. Unlike with the potion, lying while under the candle’s light (having succeeded by 10 points or more on the saving throw to answer a question) carries no danger of altering the subject’s true beliefs. However, also unlike the potion, every failed save imposes a cumulative −1 penalty on subsequent saves against the same candle, while every successful save grants a +1 bonus to same.

The zone lasts for 1 hour, as the candle burns. If the candle is snuffed before that time, the effect is canceled and the candle ruined.