"The source is the late Delhi Univ history professor K.S. Lal. But his claim is not that 80 millions were physically killed by the invaders/occupiers, merely that 80 million have gone missing from the extrapolated demographic development (itself already a very risky guess) during that period. This includes people killed, but also the effect of lawlessness and famines which he claims were triggered by the extremely high taxation and other deliberate poverty-promoting policies of the Delhi Sultanate."