Hiranyaksha in the form of a boar had abducted the earth, Bhumi, and taken her to Patala, one of the netherworlds. On the way his tusks touched Bhumi, and she became pregnant with Naraka. Vishnu came in the form of a boar, Varaha, and killed Hiranyaksha. Vishnu rescued Bhumi and had pity on her baby asura son, Naraka. Vishnu gave him the divine weapon Narayanastra and a boon that no one could kill him but Vishnu.
This is the formula for certain tragedy in the Puranas—but it was usually caused by the conceit of Brahma giving such a boon. Narakasura used his boon well and caused a lot of suffering among the devas (gods) as he was growing up. The Mahabharata stated that Naraka raped Kasheru, daughter of Tvashtha, then abducted 16,100 maidens and brought them to his harem. No one could defeat him. Finally Naraka was killed by Siri Krishna, who was of course an incarnation of Vishnu.
