Padmapurana details the legend associated with the temple.As per Hindu legend, Arjuna, one of the five Pandavas from the epic Mahabharatha reached this place on a pilgrimage. He was thirsty and sought water from sage Agastya who was doing penance.
There was no water with the sage and all places around. Agastya realised that it was the trick of god Krishna, who was the Lord of Arjuna. He advised Arjuna to pray Krishna, who did accordingly. Krishna was pleased with the prayer of Arjuna and offered him a sword.
Arjuna dug the land with the sword which resulted in a gush of water resulting in the Katka Pushkarani, the temple tank. Arjuna is depicted in the temple sporting a sword, following the legend.
Krishna also appeared to Arjuna as Parthasarathy and initiated his knowledge to understand who he actually was. Since Parthasarathy initiated education here for Arjuna, the place is called Parthanpalli. (Parthan + Palli, meaning school).
Image of shrine of Agasthiyar
The legend of all the eleven temples of Thirunangur are closely associated with each other. As per legend, the Hindu god Shiva started dancing in fury at this place after the death of his consort Uma due to the yagna (sacrifice) of Daksha. Each time his lock of hair touched the ground, there were eleven other forms of Shiva who appeared.
The celestial deities were worried that if the dance continues, it would result in decimation of entire creations. They prayed to Vishnu for help, who appeared at this place. On seeing Vishnu, Shiva's anger was reduced and he requested Vishnu to appear in eleven forms like he did. On his request, Vishnu appeared in eleven different forms at Tirunangur.