Shani Dev is the planetary deity of Saturn and is one of the Navagrahas. He is the son of Sun God (Surya) and Chhaya (Shadow), and the brother of Yama (God of Death).
Despite his fearsome reputation, Shani is actually the god of justice and karma. He rewards good deeds and punishes bad ones. His gaze is said to bring challenges that ultimately lead to growth.
Shani Sade Sati (7.5 years of Saturn transit) is a significant period in Vedic astrology, and worship of Shani helps mitigate its effects.