The demi-Gods were the sons of sage Kashyap and Aditi. One of them was Surya, the Sun God. Surya had three wives - Samjna, Rajni and Prabha. Rajni and Prabha gave birth to Revata and Prabhata respectively. However the Solar Dynasty originated from Surya and Samjna.
Surya and Samjna had two sons and a daughter. The eldest son was Vaivasvata Manu and the remaining two children were the twins Yama and Yamuna. Samjna was unable to bear the Sun’s radiance for long and she created a woman out of her own body who was identical to her. She was Chhaya, the Shadow. Samjna instructed Chhaya to look after her husband and children and to never to reveal her true identity. She then left for good.
Chhaya continued to live as Samjna and had four children. Savarni Manu and Shani were the sons and Tapti and Vishti were the daughters. Chhaya began to give step-motherly treatment to Samjna’s children. Vaivasvata was mature and did not mind but Yama resented this and once raised his foot to kick Chhaya. Chhaya pronounced a curse on Yama, who went crying to Surya and said that he doubted that Chhaya was his real mother.
That night Surya forced Chhaya to reveal the truth. Hie then went to his father-in-law, Viswakarma who was the architect of the Gods. Viswakarma told Surya that Samjna had assumed the form of a mare and lived in the desert, Maru. He asked Surya to forgive Samjna because she ran away not out of ill will but because she was unable to bear Surya’s radiance. Viswakarma then shaved off some of Surya’s energy so that people could look at him. With the excess energy he made some weapons for the Gods such as Vishnu’s chakra or discus and Shiva’s trishul or trident. The only parts of Surya’s body that were not modified were his feet. Hence generally Surya’s feet are not depicted.
Surya too assumed the form of a horse and joined Samjna in the desert. As horses they had twin sons, Nasatya and Dasra. Collectively they are known as Ashwini Kumar or the Horse Princes. They became the physicians to the Gods. Surya and Samjna then resumed their normal forms.
Meanwhile the other children of Surya were growing up as well. Savarni Manu decided to spend a lifetime in meditation on Mount Sumeru. Shani became a planet and Yamuna and Tapti became rivers. According to the epic Mahabharata, Tapti married King Samvarna and gave birth to Kuru. From Kuru that dynasty came to be known as Kauravas. Yama went to a place of pilgrimage named Gokarna. There he prayed to Shiva for a thousand years and became the God of Death.