Birth of HariHaraPutra
These sages discovered that the sages living in Tharukavanam, had come to the conclusion that pain and pleasure are the direct results of one's own action in this as well as previous lives and the Gods cannot do anything about it and hence all prayers were futile. Their philosophy of life spread throughout the world. Because of this mistaken notion, the world reeled under a lot of misery.
Saddened by the developments, the sages who came to Tharukavanam, approached Lord Vishnu and prayed that he personally get involved and set things right. Lord Vishnu suggested that all of them approach Lord Shiva to find a solution.
Meanwhile, all the devas, unable to bear the torture of King Kalikeyan, under the leadership of Lord Indira approached Lord Brahma and sought a solution to their problems. Lord Brahma suggested that all of them approach Lord Shiva to find an end to King Kalikeyan's rule. The sages with Lord Vishnu and the devas with Lord Brahma approached Lord Shiva with their problems and sought the intervention of Lord Shiva to set things right.
Lord Shiva's solution
Lord Shiva told Lord Vishnu to take the form of Mohini and go to the sages residing in Tharukavanam, while he will go as a Bhikshadana (a nude beggar). As per directions of Lord Shiva, Lord Vishnu went to Tharukavanam in the form of Mohini, the divine enchantress. The sages in Tharukavanam, were so enamoured by Mohini that they started to follow her.
Lord Shiva in the form of a Bhikshadana approached the wives of these sages who were arrogant in the thought that there were no other women on earth who were as devoted to their husbands as them. However, the moment they saw Lord Shiva in the form of a Bhikshadana, they were so lustful that they started to follow him without even noticing that the dress they were wearing had slipped and fallen to the ground. Lord Shiva slowly made his way to where Lord Vishnu was in the form of Mohini.
When Lord Shiva reached the place where the sages were, the sages realized that their wives were enamoured by this Bhikshadana (little realizing that this was Lord Shiva himself). In anger all of them teamed up together and performed a black magic ritual from which came a Tiger, an Axe, a Deer, a snake, the deity of death and disease. The sages commanded each of these to attack Lord Shiva in the form of Bhikshadana. Lord Shiva killed the tiger, skinned it and wore the skin around his hips like a skirt. He held the axe and the deer in his hand, wore the snake as his necklace and pushed the deity of death and disease at his feet and danced over him. The sages finally called upon Lord Agni the deity of fire and got a drum from him which they commanded to attack Lord Shiva. Lord Shiva took the drum in his hands. Having been unsuccessful in all their attempts, the sages were confused.
Lord Shiva then took his original form and told the sages that he played this whole drama to make them realize their mistake and that he was not angry with them.
Lord Vishnu in the form of Mohini wanted to see Lord Shiva in the form of Bhikshadana once again and fell in love with Lord Shiva. Out of their union was born a child - Sri Hariharaputra (Hari = Vishnu; Hara = Shiva; Putra = Son).
While the child Hariharaputra was growing up, Lord Shiva wished to have a place made for him and called for the architect of the Devas to construct a beautiful city for his son. As per directions from Lord Shiva, the architect constructed a beautiful city called Tejovathi. Lord Shiva then appointed Hariharaputra as the ruler of the whole universe (and hence the name Saasta or Mahasaasta) and commanded the four Vedas to take the form of an elephant and become Mahasaasta's vehicle.