Bhagavad Gita 8.25धूमो रात्रिस्तथा कृष्णः षण्मासा दक्षिणायनम्।तत्र चान्द्रमसं ज्योतिर्योगी प्राप्य निवर्तते ॥

This is the Pitriyana or the path of darkness or the path of the ancestors which leads to rirth. Those who do sacrifices to the gods and other charitable works with expectation of reward go to the Chandraloka through this path and come back to this world when the fruits of the Karmas are exhausted.Smoke, night time, the dark fortnight and the six months of the southern solstice are all deities who preside over them. They may denote the degree of ignorance, attachment and passion. There are smoke and darkcoloured objects throughout the course. There is no illumination when one passes along this path. It is reached by ignorance. Hence it is called the path of darkness or smoke.