By Swami Harshananda
Sometimes transliterated as: Sarpayaga, SarpayAga, Sarpayaaga
Sarpayāga literally means ‘sacrifice of serpents’.
A yāga is a very powerful instrument for achieving any result one wants, good or bad. The king Parīkṣit had been cursed by the young sage Sṛñgi to die by snake-bite, since the king had humiliated the sage Samīka by putting a dead snake round his neck. Ultimately the king died by being bitten by Takṣaka, the lord of snakes in the nether world.
Janamejaya, the eldest son of Parīkṣit who succeeded him, decided to take revenge on Takṣaka and the entire race of snakes and serpents by performing the Sarpayāga. As the yāga started and progressed, innumerable snakes and serpents were drawn irresistibly into the fire and perished. Meanwhile Takṣaka had taken refuge in Indra, the king of gods and hence he could not be easily drawn into the sacrificial fire. Knowing this through their power of intuition, the ṛṣis who were performing the sacrifice, changed the mantra to include Indra also. Indra also was being pulled by the power of the mantra, he abandoned Takṣaka and escaped. But before Takṣaka actually fell into the fire, his life of the surviving snakes, was saved by the timely intervention of a young sage named Āstīka 
- The Concise Encyclopedia of Hinduism, Swami Harshananda, Ram Krishna Math, Bangalore