Harihara is a combination of the gods Vishnu as Hari and Shiva as Hara. This combination is usually associated with the conquest of Mahishasura and regaining the Amrita, the nectar of immortallity. In other stories the union of Hari (as a female) and Hara led to the birth of Ayyappan. When Vishnu demonstrated that form, Siva is said to have been so enchanted by it that he ran and embraced Vishnu in that feminine form. The energy of Siva which was released at that moment became the reason for the birth of Sasta or Harihara Putra (son of Vishnu and Siva), as he is called. http://www.dlshq.org/religions/theology.htm Sasta is called Ayyappan in the South.