Flickerish

Flickerish is a code language invented by scholars who worshipped Madam Midnight in Their aspect of the Candlekeeper. The language uses a candle's flame, and a series of mirrors, fans, and shutters to create complex patterns of flickering light, allowing practitioners to talk across great distances.
Flickerish was originally intended as a needlessly complex language to participate in a display of intellectual superiority by following careful call-and-response phrases, but eventually expanded to a wider call-and-response passtime for scholars across the Vekhen Empire. However, during the End Times, the language was adopted by the military elite as a form of long-range communication to co-ordinate troop movements en masse. This usage was tightly controlled to prevent intelligence being stolen by spies, and is considered one of the cornerstones of the Prophet's strategy.