Many were the sages doing penance on Mount Kailash. One of them, Sundaranatha, wanted to meet Sage Agastya, and since Agastya was in south India, he proceeded there. He worshipped at the Siva temples in Kedarnath, Nepal and Srisailam, and then reached Kalahasti, in present day Andhra Pradesh. From there he went to Thiruvalangadu, Kanchi and Chidambaram, and after worshipping in all these places, Sundaranatha reached Thiruvavaduthurai, where he stayed on and worshipped the Lord in the temple. One day as he was approaching Sathanur village, he saw a man grazing cows. His name was Moolan. He treated the cows with affection, and they reciprocated his love. But even as Sundaranatha was watching, Moolan dropped down dead. The cows stood around him and wept. Moved by their plight, Sundaranatha, using his powers, quit his body and entered into Moolan's, which thus came back to life, and the cows were led back to the village.
Moolan's wife thought that her husband was back and asked him to come home, but he refused. The next day, the village elders assembled, and went in search of Moolan. They found him under a tree meditating. Since the soul of Sundaranatha had entered his body, Moolan now glowed with divine effulgence. The village elders realised that he would no longer be interested in family life and they told his wife to reconcile herself to this. So she returned home.
As Sundaranatha, who was in the body of Moolan, meditated, he realised that he had been sent to south India by none other than the Lord Himself so that he may present Vedic knowledge in Tamil. So Moolan, who had the spirit of Sundaranatha in his body, now became the Nayanmar, Thirumoolar, and composed verses, giving Vedic principles in Tamil, said R. Narayanan.
Although Sundaranatha had come to south India to meet Sage Agastya, the Lord's purpose in sending him was to get him to present Vedic principles in Tamil. The Lord has His plans and it is His plans that materialise. There is nothing any human being can do to alter His plans even one bit.