Decline and end of NALANDA
Evidence in literature suggests that in 1193, the Nalanda University was sacked by the fanatic BAKHTIYAR KHILJI, a Turk. Muslim conquest in India is seen by scholars as one of the reasons of the decline of Buddhism in India. The Persian historian MINHAJ-I-SIRAJ, in his chronicle the TABAQAT-I-NASIRI, reported that thousands of monks were burned alive and thousands beheaded as Khilji tried his best to uproot Buddhism the burning of the library continued for several months and "smoke from the burning manuscripts hung for days like a dark pall over the low hills."
The last throne-holder of Nalanda, Shakyashribhadra, fled to Tibet in 1204 CE at the invitation of the Tibetan translator Tropu Lotsawa (Khro-phu Lo-tsa-ba Byams-pa dpal). In Tibet, he started an ordination lineage of the Mulasarvastivadin lineage to complement the two existing ones.
When the Tibetan translator Chag Lotsawa (Chag Lo-tsa-ba, 1197–1264) visited the site in 1235, he found it damaged and looted, with a 90-year-old teacher, Rahula Shribhadra, instructing a class of about 70 students. During Chag Lotsawa's time there an incursion by Turkish soldiers caused the remaining students to flee. Despite all this, "remnants of the debilitated Buddhist community continued to struggle on under scarce resources until c. 1400 CE when Chagalaraja was reportedly the last king to have patronized Nalanda."
D.C. Ahir in his book 'Buddhism Declined in India : How and Why? - B. R. Publishing (2005)' considers the destruction of the temples, monasteries, centres of learning at Nalanda and northern India to be responsible for the demise of ancient Indian scientific thought in mathematics, astronomy, alchemy, and anatomy.