Shani deva is one of the Nine planets. he is Saturn the seventh planet.
Shani Dev is son of Surya (sun) and Chhaya. he is the elder brother of Yama , the lord of death. Both the son of Surya are lord of justice.
Shani is the lord who rewards as well as punishes one according to his deeds in his life , Where as his brother Yama , the lord of death do the same after one’s death.
Story of Shani dev’s birth:
when Shani was in womb of Chhaya, who was a great devotee of Lord Shiva . she did not take much care of herself and was deeply involved in the worship of Lord Shiva , as a result Shani was born with a Dark complexion. Surya , the father of Shani was not happy with this because he was the sun , the lord of light and brightness , so he did not accept Shani as his son . when Shani opened his eyes his father went into eclipse . Shani considered his father as his enemy , he did Tapasya (penance ) to please Lord Shiva . Shiva gave him a boon to be the most powerful among all the planets including his father Surya. Shani defeated his father to avenge his and his mother’s insult.