Special candle found generated in Gehennom. While one is lit on a level, it suppresses spawn rates and makes the difficulty about what it is now (without the candle lit monsters will generate at a much higher difficulty). They should be fairly nonrenewable, perhaps wishable but not polymorphable. This limits your duration of time in “easy Gehennom” before it gets harder. The candles work the same outside Gehennom, but are less useful (maybe they specifically suppress demon spawns).