The goddess Samjna created her replica, Chaya, in order to fool her husband, the sun (Surya). After having three children by Surya (Manu, Yama, and Yami), Samjna could no longer stand the heat of his affections. So she created Chaya to take her place. Things went well for a while, and Chaya gave birth to Sani (the planet Saturn), a Manu (first human of a cosmic age) known as Savarni, and a river goddess, Tapati. Chaya treated her own children so well that Yama, who had been fooled as completely as his father Surya, tried to kick the woman he believed was his mother out of jealousy. Chaya cursed him so that the foot that he raised against her would forevermore be afflicted with sores and worms. That was certainly fitting for the future god of death. Surya suddenly realized that she could not be the real mother of Yama, so he searched for his real wife. He found Samjna and brought her home.
Another version makes Chaya the daughter of Visvakarma and therefore the sister of Samjna.