The asura princess Mahishi was burning up with anger at the trick the gods had pulled on her brother, the asura king Mahishasura. As Mahishasura was blessed with invulnerability to all men, the gods had sent goddess Durga, to fight and kill him. Thus, Mahishi began performing a fearsome set of austerities, and pleased the creator god Brahma. She asked for the boon of invulnerability, but Brahma said it was not possible so Mahishi planned and asked invulnerability to all men except by the son of Shiva and Vishnu (Both are male & no possibility to give birth to any one). He granted her the boon of ruling the universe and being invulnerable except by the son of Shiva and Vishnu. Since such a person did not exist, she thought she was safe and began conquering and plundering the world.