(SkandaPurana- Kashi khanda)
Lord Surya was married to Sangya(Samjna)- the daughter of Vishwakarma. She was extremely beautiful and very loyal to her husband. Lord Suryadeva was very much pleased with his wife. But, Sangya(Samjna) was very much upset with the tremendous radiance of Lord Surya. She couldn't tolerate his glare! Thus, she had decided to do hard penance either to increase her radiance or to diminish the glare of her husband- Lord Surya. By that time they were blessed with three children- a daughter, Yamuna, and two sons- Vaivasvata Manu and Yama Deva. Sangya(Samjna) loved her children very much. One day, she thought of going away from her husband for a few days to perform penance. Thus, she created her own shadow (Chaya) and named her Savarna.
As Savarna was formed, joining her hands she asked, ‘O goddess! How can I help you? I am ready to do.’
Sangya(Samjna) said, ‘O beautiful Chaya! I will go to my father’s house for a few days. O noble lady, on my behest, you stay here to take care of my children playing the role of their mother. And, let this be a secret! Don't let my husband have the slightest doubt on you and me. But, you should remember that you are Chaya, not Sangya. If ever any problem comes up, call me. I’ll be there. I will go and perform hard penances to reduce the glare of my husband or I shall increase my brilliance.’
With humility, Chaya said, ‘Let it be so, O lady of sweet smile! Go to your father's abode happily! I will carry out your command perfectly well!’
Thereafter, Sangya(Samjna) went to her father’s abode without informing her husband and told her father about her incapability to tolerate the glare of her husband. She also mentioned her desire to do hard penances. But, her father, Daksha became furious and rebuked her. He said, ‘O Sangya! If without being invited a daughter comes to her father's house then both the daughter and father are cursed! So, go back to your husband immediately.’
Hearing the words of her father, Sangya was very much worried thinking about what would happen to Chaya if she would go there right then! She thought, ‘There is no freedom to a woman at any time! She always remains at the command of her father, husband, and son from her childhood, youth, and even old age! If I would not go back to my husband, my father would get angry, if I stayed here Lord Surya would become more fiercer than my father! He will become terrible. And it would be like the saying- that one is holding the burning coal with his or her own hands.’
Thus, she mulled over the situation, changed her form to a mare, and went to Northern Kurus to perform penances. She spent her time only grazing dry grass and keeping her husband in her mind all the time.
Thinking of Savarna as his real wife, the Sun God happily spent his time with her. Later, Chaya gave birth to three children- Savarni Manu, Shani Deva, and Bhadra- Tapati. When Shani was in his mother's womb, Chaya was deeply engaged in penance. She was praying to Lord Shiva. She was so much engrossed in Tapasya that she continued it under the blazing Sun forgetting everything including her food. Thus, the divine effect of her worship fell upon the child inside her womb. When Shani was born, he became an ardent devotee of Lord Shiva. He was born with a dark complexion. Beholding his color, Lord Surya started doubting Chaaya if Shani was born to him. He misbehaved with Chaya and Shani too. This raised the anger of Lord Shani. He gazed at his father with a cruel look. Due to his look, Sun-God himself burned and became black. His chariot stopped moving and the steeds of the chariot lost their lustre.
Later, he worshiped Lord Shiva to know the truth. At last, Lord Shiva told him everything. After knowing the truth, Surya Dev realized his mistake and apologized. Then both father and son started loving each other. Lord Shani became a great devotee of Lord Shiva and he learned all the knowledge from Him only!
With the divine effect of his mother's penance, Shani became too powerful from the day of his birth! No one in this universe can escape from Shani Dev’s influence! It is said- that Lord Shiva (Gurudev of Shani) was not exempted from his cruel gaze neither was a king like Vikramaditya!
According to Vedic astrology, every planet is associated with a deity. Thus, planet Saturn is associated with Lord Shani Dev! He is the lord of Karma. He is known as the symbol of righteousness and justice! He gives accurate Karmic rewards to each one according to their good and bad deeds done in their past life! One has to face Shani Dev in his lifetime. Thus, Shani was born to Savarna and Aditya (Surya Dev).
Shani went to Varanasi and installed a Shivalinga. He performed hard penances praying Lord Sankara. Later it was called Sanaiscaresvara. It is said- that one who visits Varanasi worshipping Sanaiscaresvara on Saturday can ward off the oppression caused by Saturn.
One should worship Lord Hanuman to get rid of the bad effects of Shani Dev!