Wikipedia
Dharmasasta or Dharmashasta, is a Hindu deity adored all over India particularly in Kerala. Lord Dharmasasta is the son of Lord Siva and Mohini—the female transformation of Lord Vishnu depicted in the famous story of 'Palazhi Mathanam', the churning of the ocean, in epics and puranas. He is addressed as Hariharasuthan, which means son of Hari (Vishnu) and Hara (Shiva).
Dharmashasta is worshiped by people belonging to both Saiva and Vaishnava denominations. Social historians view the concept of 'sasta' as an attempt to remove the conflict between the two cults by amalgamating them. This godhead is widely adorned all over India and occupies the status of 'upadeva' or sub-deity in many temples.
Dharmasasta is depicted on a horse. He seeks bliss in hunting in forests, desires drinking water in the mid-days, has three eyes and is surrounded by the bhutaganas. The god is named "Bhutanatha" meaning the commander of bhutas (all those who are created).
Dharmasasta has two wives, Purna and Pushkala, and a son Satyaka. He is the protector of dharma. The aim of dharma is perfection (Purna) and prosperity (Pushkalatva) which are achieved by dharma and wisdom. Adherence to dharma enables one to achieve truthfulness ( Satya) that is Satyaka.
Lord Ayyappan of Sabarimala of Kerala is a major abode of this deity.
Although scholarly opinion varies, Ayyappan and Sasta are synonymously used by Keralites in local stories, rituals, ritualistic arts, folk songs, bhajans etc.
The popular belief among Keralites is that Lord Ayyappa, the adopted son of the King Rajasekara Pandiya of Pandalam in Kaliyuga, was a human incarnation of Lord Dharmasasta, the hariharasudha. He was called Manikantan by the King since he had a Bell (mani) in Neck (kanta) when the King found him as an infant at the banks of the river Pamba. Manikantan, after fulfilling the goals of incarnation, selected the Sabarimala as his place for meditation, to elucidate his dharma, the sciences of wisdom to the people of the world, to the gods and to the rishis. There he experienced his ashtasiddhis (the great knowledge about 8 prime factors of life) and rendered his spiritual knowledge to the devotees and those who seek refuge in him (Sharanam). Thus he was called Sri Sabarigirisan (one who resides in the mountain of Sabari).
Sasta or Ayyappan is associated with the graha (planet) Shani (Saturn). By pleasing the lord one can reduce the bad effects of Saturn called sanidosha. Offering Neerajanam to Lord Ayyappa on Saturdays is believed to be good for removing hardships owing to sanidosha.