Bhagavatam - Deliverance of Demon Agha
Demon Agha is the younger brother of Baka and Putana. Seeing the joyous pastime of Srikrishna and cowherds, Agha wasn't happy and wanted to avenge Srikrishna.
He resolved to kill all the cowherds along with Srikrishna. He thought once these children are killed, it is almost like killing entire residents of Vrindavan as without progeny life would turn dead.
Demon Agha assumed the form of Boa constrictor (serpent) too big to be identified. Its length was around 8 miles, stout as a huge mountain. Its mouth was as wide as a cave opening, its lips touched the clouds, tongue as broad as road, eyes resembling a wild fire.
The cowherds wondered to see the huge figure and discussed amongst themselves whether it could be a boa constrictor. If so, it would perish in the hands of Srikrishna. Saying so, they, along with their calves, entered into the wide mouth of the serpent, clapping hands and laughing loudly.
The serpent, though real, appeared fictitious to them. Srikrishna tried to restrain His companions, but they had already entered the mouth of the serpent with their calves.
The demon, whose main target was Srikrishna, waited until His entry. The gastric fire of the demon made the cowherds and their calves unconscious.
Srikrishna was anxious to save those companions and as well punish the wicked demon Agha. He entered the mouth of the serpent and speedily grew in size that choked its throat. Consequently, the serpent eyes shot out, and the vital air split opened from its crown.
Source: Bhagavatha Mahapurana