Indira, to get the backbone killed the sage and then killed the demon and incurred the Brammhathi dosha.

Demon Vruddhasura was harassing the Devas in a terrible way.  Their king Indira appealed to Lord Brahmma for protection.  He said that Indira had to get the back bone of Sage Thatheesi, make a vajrayudha weapon of it and should apply it on the demon and that alone could kill him.  Lord Brahmma also explained the secret of the strength of the back bone of the sage.  Before joining the churning the milk ocean, all the Devas entrusted their weapons with the sage.  The sage simply put all their weapons safely in his mouth and continued his penance then.  All the weapons melted and merged with his back bone making it miraculously strong.  When Indira approached the sage and expressed his purpose and wish, the sage calmly parted with his back bone.  Indira killed the demon with the weapon but incurred the Brahammahathi dosha the impact of the highest sin.

Indira approached his Guru for solution.  He advised him to go to earth and perform Shiva puja and that he would find a Shivalinga at a place under a Vanni tree made of drops of nectar.  Guru also advised Indira to create a spring and perform abishek to Lord with the water then.  Indira followed the advice of the Guru scrupulously and freed himself from the sin.  As Indira worshipped Lord here, the place was named Indirapuram and also Kottur as his white elephant Iravadham drew a line (kodu in Tamil) in the place.