The killing of Demon Vritrasura is an important event in the Vedas. Sage Agastya or Agasthiar helped Indra in killing the all powerful demon.
Demon Vritrasura was unimaginably huge and he conquered the three worlds. The Devas were easily overpowered by the Demon. They lost heaven and earth to the Vritrasura.
Devas approached Lord Vishnu to find a solution. Vishnu advised Devas to make a weapon out of the backbone of the powerful Sage Dadhichi. The sage renounced his body for the good of the universe. The great Vajrayudha was created from the backbone of Sage Dadhichi by Vishwakarma, the artisans of gods.
When Vritrasura heard about the invincible weapon, he disappeared from the universe. Indra searched for him everywhere.
When he was unable to locate Vritrasura, Indra approached Brahma. Brahma advised Indra to take the help of Sage Agastya.
Sage Agastya, one among the Saptarishis, used his yogic powers and located Vritrasura. The powerful demon was meditating on a huge mountain under the ocean.
Indra did not have the power to reach the Demon who was protected by the great ocean.
Sage Agastya used his yogic powers and drank the waters of the ocean. As the ocean began to recede into the mouth of the yogi, Vritrasura appeared sitting atop a mountain.
In the battle that followed, Indra beheaded the demon using the powerful Vajrayudha.