Qutub-ud-din Aibak was finally purchased by the ruler of Ghor in central Afghanistan, Sultan Muhammad Ghori. Qutub-ud-din Aibak gradually rose to the rank of a Commander and became a trusted slave of Sultan Ghori. The conquests of northern India were executed mainly by Qutub-ud-din Aibak, which helped Ghori to consolidate his position there. Gradually, as Sultan Ghori concentrated on Central Asia after 1192, he was given the independent charge of the conquests in India. In India, Qutub-ud-din Aibak is infamous for massive destruction and plunder of several Hindu temples, whose wealth he looted away. His lieutenant, Bhakthiar Khilji, following in his master's footsteps ravaged the famous university of Nalanda, killing all the monks and scholars there, as well as burning the huge library down. It is said that the fires raged for 3 months, such was the voluminous material being burnt.