Which demoness was killed by the infant Krishna?

The demoness killed by the infant Krishna was Putana.

Putana is a rakshasi, or demoness, who appears in stories of Krishna’s childhood. The tyrant Kamsa, Krishna’s maternal uncle and enemy, sent her to destroy the infant before he could grow into the figure prophesied to defeat him. Putana disguised herself as a beautiful woman and entered the cowherd settlement where Krishna was being raised by Yashoda and Nanda.

She offered to breast-feed the child after applying poison to her breast. Krishna, however, drank not only the milk but also her life force. Putana cried out and resumed her enormous demonic form before dying. The story is especially distinctive because Krishna’s act is described as both the defeat of a killer of infants and, in many devotional interpretations, the granting of maternal status or liberation to a being who had physically nourished him.

Putana is sometimes confused with other demons defeated during Krishna’s infancy, including Bakasura, Aghasura and Trinavarta. Those figures appear in separate episodes. Putana’s defining detail is the poisoned nursing disguise.