[zozbot234]

The Mongolic ("Islamic") invasions from Central Asia starting in the late middle ages were quite comparable in their violence. And they still count as "west" from an Indian POV since Central Asia is directly west of the Indian subcontinent. (Note that these Mongolic invaders were, by and large, merely converts to Islam, and that a very comparable dynamic also caused the end of the Golden Age of Islam, around that same time.)

Gautama Buddha was against any rigid interpretation of the varnas, but he was not unusual in that - this was close to Hindu orthodoxy even at the time.