Vartraghnī Saṅgama is the holy confluence where the river Vartraghnī joins the Śābhramatī. Here Indra did tapas (austerity) and won the favour of Bhagavān Śiva, who gave him the vajra (thunderbolt) to kill Vṛtra. The Padma Mahā Purāṇa tells of it in the Uttara Khaṇḍa, Chapter 168.

Indra, Vṛtra and Bhagavān Śiva

The war between Indra and Vṛtra lasted ten thousand and ten hundred years. At last Indra, defeated, made a truce with Vṛtra and took shelter with Bhagavān Śiva. At the holy confluence of the Vartraghnī with the Śābhramatī he did tapas to please Śaṃkara. Bhagavān Śiva appeared to him in the sky.

Ash fell from the body of Bhagavān Śiva, and from it a liṅga named Bhasmagātra (“body of ash”) was made on the bank of the Kaśyapī. Brahmā installed it as Bhūteśvara-Bhasmagātra. By its sight alone the pāpa (sin) of brahmahatyā (killing a brāhmaṇa) is destroyed.

Bhagavān Śiva was pleased with Indra and gave him the thunderbolt, saying: “By means of this thunderbolt you will quickly kill Vṛtra.” Indra went out and killed Vṛtra on the shore of the ocean. After the victory he did an aśvamedha (horse yajña) here and became free of pāpa.

The Brahmahatyā from Vṛtra

Vṛtra was an asura, yet he counted as a brāhmaṇa. When Indra killed him, Brahmahatyā, the pāpa of killing a brāhmaṇa, came out of Vṛtra’s body in a fearsome female form. She was black and tawny, wore a garland of skulls, was smeared with blood and smelled of fish. She seized Indra by the neck. He hid in a lotus fibre for many ages.

Then Brahmā divided Brahmahatyā among four: Fire, Trees, Apsarās and Waters. Each took a share, with a condition for its release. Freed from her, Indra went back to his capital, did the aśvamedha at Vartraghnī Saṅgama, and became free of pāpa.

Origin of the name

The river Vartraghnī takes her name from what happened here. Vartraghnī means “slayer of Vṛtra” (Vṛtra and ghnī). The name keeps the memory of Indra’s purification and of the defeat of Vṛtra.

Merit of bathing here

Bathing at Vartraghnī Saṅgama gives the fruit of ten aśvamedhas.
A śrāddha here, with piṇḍas (rice balls) made of powdered sesame, purifies seven earlier and seven later generations.
A man who bathes here and worships Gaṇanāyaka is not overcome by dangers and is not left by glory.

Sources

Padma Mahā Purāṇa, Uttara Khaṇḍa, Chapter 168