In terms of the scale, I voted for the Ghengis Khan and his Mongolian cavalries' conquest over almost the whole Eurasia continent.
Years ago I read a population document concerned with him, which indicated before he put his feet into Pan-China's territory (all-together 4 nations Song, which is a Han-dominated state, Xi Xia, Liao, which was ended by the last one, Jin ), the whole population was roughly 150 million. And after he finished his job, I mean only the mindless conquest in China, there were only 50 million.
Mongolian army was notorious for the whole-scale no-one-left massacres of the cities whose garrisons and residents objected to surrender.
Undoutebly, it was the most "horrendous" event, although just in terms of the scale, which is also an undoutebly narrow sense.