Agastya - Maharshis of Ancient India

Agasthya was one of the greatest Maharshis and was born with Mahavishnu's blessings. He was a great devotee of Lord Sankara and Lord Shanmukha. He installed a number of Shivalingas and was the first sage to visit the southern part of India.  Lord Brahma asked Vasishta to return to the world to resume the good work. Thus, two Maharshis were born through Mithra and Varuna, Agasthya being the first and Vasishta following him.
Agasthya knew the secrets of the Vedas at birth and was an expert in the use of weapons.  Two demons, Ilvala and Vatapi, were the scourge of several Brahmins. Agasthya saw through Ilvala's devilish plans and simply put an end to Vatapi by digesting him! Seeing his pithrus hanging head downwards from a big tree because he was not still a gruhastha, he agreed to get married so that they could reach heaven.
Agasthya was able to swallow the water of an entire sea so that Kaleyas, the demons who hid at the bottom of the sea, could be got rid of. He could convince the Vindhyas to stop growing, something which no one could even believe, let alone achieve. It is because of Agasthya that Nahusha turned into a huge python, thus saving Sachi Devi from marrying him even while her husband was alive. Agasthya was so powerful that he alone was able to stop the tilt of the Himalayan Mountain Range.
All the guests had assembled at the southern part of the range making it heavy on one side - this was when they came to witness Lord Siva's marriage to Parvathi. Lord Siva himself asked Agasthya to go to the south and stop the tilt. In fact, as the sage was disappointed, the Lord bestowed on him the divine power to witness the celestial marriage from his position in the south.  On the advice of Agasthya, Lord Rama chanted the hymn, 'Adithya Hridaya' in order to kill Ravana and then performed Aswamedha yaga for the welfare of the world.
Agasthya's curse was so powerful that he did not spare Kubera, the Lord of Wealth. Kubera had ignored when his friend, Mahiman, had spat on the sage's head from the sky. Again, king Indradyumna was able to reach the lotus feet of Lord Narayana because of the sage's curse. The sage was known to assume Devendra's role for the sake of protecting people on earth.  Agasthya's contribution to Tamil is invaluable.
He is worshipped in many temples in Tamil Nadu. Sri Lalitha Sahasranamam is a gift to us from the sage in addition to a number of scholars whom the sage gave to the world.