The Brahmanda puranam, kshetra kandam details the purana relating to this divya desam. In Treta yugam Sibi Chakravarti was ruling Ayodya. At this time he had heard of the attrcocities of Ravana and assembled an army to wage a war and defeat him. He and his army reached Neelikavanam near Svetha giri. They all were resting for a while, when the king noticed a white boar, the color that was unusual for the region. So he wanted to catch the boar and folowed it. The boar went into a bush and suddenly disappeared. When the king looked past the bush, he found Bhoomi devi and Markandeya rishi performing tapas and no sign of the boar.

At this time the king was not sure what was going on. He heard an Asareeri which said "O Sibi Raja! I will be born in course of time as a son of Dasaratha. I will destroy Ravana at that time." The king was not sure where the voice was comming from. He had his doubts and again another asareeri mentioned that he should construct a temple for Perumal at Tiruvellarai, and Perumal will appear there in order to fulfill the promise he had given to Thaayaar. It is said that once thaayaar expressed desire to have the pleasure of ruling the world created by Perumal. Perumail, in turn, promised that She would be given an opportunity to do Sengol Paripalanam at Tiruvellarai when Perumal would be descending to the earth in response to the penence of Markandeya maharishi and Bhoomidevi.

Latter when Markandeya rishi heared the incident from Sibi Chakravarti, he mentioned that the white boar was none other than Perumal Himself in His Varaha avataram and the King was lucky to have had the darshan. He requested the King to stay in Tiruvellarai and build a temple for Perumal. He also assured him that Perumal himself would soon take the Rama avataram and the King of Lanka would meet his end at the hands pf Perumal Himself.

Accordingly, Sibi Chakravarti built the temple at Svetha giri and installed the Perumal in the archa form. The Perumal here is also known as Svetha Girinathan. There are 5 theerthams inside the temple itself. About 13 miles South east of Svetha giri, there is a village known as Pankaja selvi. This village is on the Samapuram-Tiruchy road and has lands registered in the name of Thaayaar. The residents of this village even today are honoured during Panguni Uttiram.








Source:
1. 108 Vasihnavaite Divya Desam by Miss M. S. Ramesh.