Shani, or Saturn, is one among the Navagrahas or the nine planets associated with Hindu Religion and astrology. Shani Dev is born of Surya (Sun God) and Chhaya. The story in Hindu Scriptures regarding the birth of Shani Dev is very interesting. Lord Vishwakarma, the architect of the universe, married off her daughter Sangya to Surya.
Surya was considered deserving in all aspect to marry Sangya. The couple had three children named Manu, Yama and Yamuna. Now Sangya was very delicate so she could not endure the splendor and heat of the sun for long period.
To find a solution to this she created a replica of herself from her shadow (Chhaya) and asked to take her place as the wife of Surya. Sangya then went to Uttarakusha Mountain
The replica Sangya possessed all the qualities of the original one and therefore Surya failed to notice the change. Soon Chhaya gave birth to a child who was of black in complexion - Saturn.
Surya could not believe that it was his son due to the black complexion and therefore did not accept Shani as his son. A distressed Chhaya revealed the truth to Surya that she was not his real wife but was only a shadow of her.
Lord Surya then became indifferent to both Chhaya and Shani. This neglect made Shani cruel and stubborn from childhood.