In the middle of the fourth century of the Christian era, something resembling a volcanic eruption took place in the history of the human race. The Hūnas, like a varitable stream of lava, issued from their homeland on the northern shores of the Caspian Sea and spread over Europe and Asia. Homeless and lawless, they rodetheir horses awake and asleep. Their fierce yells spread terror wherever they were heard. They engaged all the civilized peoples of the world in fearful cataclysmic wars; and, wherever they could, they killed, destroyed, burnt and devastated with demoniac ruthlessness. In Europe, Attila the Hun brought about the downfall of even the powerful Roman Empire. About A.D. 455, the Hūnas began to enter India. Emperor Skanda-gupta drove them back by a supreme effort. Twelve years later, he died. The outposts of the empire, already weak, could offer no further resistance. The barbarian hordes, after passing through Persia and destroying the Kushāna rulers of the north-west, began to pour into India. A war of succession appears to have followed the death of Skanda-gupta, weakening the empire in the hour of its danger. Next in order of succession, five emperors including Narasimha-gutpa Bālāditya, between A.D. 500 and 570, held precarious sway over parts of the empire, which in spite of its decadence was still a name to conjure with. Many parts of the empire outside the bounds of the compact core of the empire became independent. In Saurāshtra, a province of the empire, the Maitraka general practically threw off his allegiance on the death of Skanda-gupta. By A.D. 512, the Hunas under Toramāna, overran north India up to Eran in the Saugar district of Madhya Pradesh. Toramāna's son, Mihirakula, a veritable terror, spread fire

Who was the imperial ruler of India in the middle of the fourth century A.D?