Bihar was largely in ruins when visited by Hsüan-tsang, the famous Buddhist monk from China, and suffered further damage at the hands of Muslim raiders in the 12th century. With the advent of the foreign aggression and eventual foreign subjugation of India, Bihar passed through very uncertain times during the medieval period. Muhammad Ghor attacked the this regions of the Indian subcontinent many times. Muhammad Ghor’s armies destroyed many Buddhist structures, including the great Nalanda university.
The Buddhism of Magadha was finally swept away by the Islamic invasion under Muhammad Bin Bakhtiar Khilji, one of Qutb-ud-Din’s generals destroyed monasteries fortified by the Sena armies, during which many of the viharas and the famed universities of Nalanda and Vikramshila were destroyed, and thousands of Buddhist monks were massacred in 12th century C.E.
During the medieval period, Bihar saw a brief period of glory for about six years during the rule of Sher Shah Suri, who hailed from Sasaram. Sher Shah Suri built the longest road of the Indian subcontinent, the Grand Trunk Road, which starts from Calcutta and ends at Peshawar, Pakistan. The economic reforms carried out by Sher Shah, like the introduction of Rupee and Custom Duties, is still used in the Republic of India. He revived the city of Patna, where he built up his headquarter.
From 1557 to 1576, Akbar, the Mughal emperor, annexed Bihar and Bengal to his empire and the region passed through uneventful provincial rule during much of this period. With the decline of Mughals, Bihar passed under the control of Nawabs of Bengal. This period saw exploitation at the hands of the rulers in the form of high taxes, but the Nawabs of Bengal also allowed trade to flourish in this region. Some of the greatest melas of the Indian subcontinent, viz., Soenpur Mela which is the biggest cattle fair in India, were allowed to continue and even flourish inviting traders from far and near.
Guru Nanak Dev had visited Patna, stayed at Bhagat Jaitamal’s house near in Gaighat, Patna in 1509 A.D. and later by Guru Tegh Bahadur along with Patna his family in 1666. The 10th and the last Guru of Sikhism, Guru Gobind Singh was born at Patna Sahib, Patna in 1666. Prince Azim-us-Shan, the grandson of Aurangzeb came as the governor of Patliputra in 1703. Azim-us-Shan named Patliputra as Azimabad, in 1704.
Source : Wikepedia