sastha took birth as a human baby for Hari who is perumal and haran who is siva. So Sastha called Harihara sudhan. Sudhan means son. Sastha is being called Tharaga brammam born to siva due to Vishnu maya. Tharaga brammam means God the highest. We can learn the history of sastha from Boothanatha Puranam and Ayyappan songa.