Vritraghni is a name used to refer to Saraswati River and Goddess Saraswati in the Rig Veda. The story is associated with killing of demon Vrita. Legend has it that Demon Vritra removed moisture from earth and thus making earth dry.
Due to the evil activities of Vritra, the earth became barren and living beings began to perish. There was death everywhere and no birth.
Indra subdued the demon Vritra with the weapon Vajra.
Goddess Saraswati assisted Indra in annihilating Demon Vritra.
For this divine act, Saraswati River is called Vritraghni, the feminine form of Indra’s epithet Vritrahan.