According to Hindu mythology Sun Lord is Tao Of Rich married to a beautiful lady called Samjna. They had three children namely Manu, Yama and Yamuna(Yami). It is given that Yama and Yamuna are twins. They enjoyed a pure bond of sibling love.But unable to bear Sun's dark form, Samjna flees back to her father. She creates a shadow woman of her own, Chaaya, and asks her to stay with her husband. Chaaya and Sun have a son of their own. Like any other stepmother Chaaya paid more attention her own son. On this Yama revolted. One day, Yama, unable to tolerate Chaya's cruelty any longer, stamped her foot hard. Offended by his action, Chaaya curses him and says,"Let that foot of yours fall".
Yama goes to his father and narrates the whole incident and pleads him to revert the curse. After hearing the whole story, Surya realises that Chaaya cannot be Yama's mother. Chaaya's real identity comes out. Surya then reaches Vishwakarma's place looking for his wife, Samjna. On meeting her, he comes to know that she had to flee because of his radiation. Vishwakarma with his powers reduces Surya's light.
Although, Surya could save his son, Yama, from death, he could not save him from mutilation. As a result Yama became the God of Death. While Yama is sent to the underworld, his sister Yamuna weeps over the separation from her brother. Her sorrow was so intense that Earth suffered - fires began and raged. Slowly it dawned on the Gods and Goddesses that Yami's grief was not lessening for she was stuck in time. Till that there was only day. No nights. They thought that she would calm down if night was created. And she was made the Goddess of Night "Yamini".