This scenario's a few years old now, but here's the sanitized synopsis I gave my AI assistant recently.

In this scenario, aliens arrive, transmitting a brief explanatory message revealing that the Earth is one of many manufactured and managed "farm" planets operating on a ~ten-thousand-year cycle, for the production of human slaves (used for all kinds of purposes, many of them unimaginably terrible). Within minutes of the aliens' arrival, they have taken roughly a third of Earth's population - and a critically important third for peacefully maintaining industrial civilization, too. The aliens then simply leave, but not without leaving behind a powerful alien AI system tasked with spending the next few generations sadistically overseeing the gradual but complete dismantling of technological civilization and historic records. Within a couple of centuries, the Earth is effectively reset to a prehistoric state, from which will develop an entirely new advanced global civilization - just in time for the next long cyclical harvest.

I note, by the way, that the critical third of the population removed consists of the overwhelming majority (say, >99%) of women under fifty - which you can imagine rather shapes the ensuing civilizational collapse and depopulation.

It's practically an abstract concern for those still on Earth compared to civilization's apocalyptic fate, but extra notes of despair are added by the alien message itself, which seemingly only exists to rub it in. That message notes:
A) vague but horrifying details about the terrible fates likely to befall those taken as slaves
B) the general constructed cycle of Earth as a farm for producing this exact scenario
C) the commonplace, low-caste nature of Earth in the universe - IE, that there is a morally repugnant and biologically diverse civilization filling much of space, and many stars hold prefabricated farm planets precisely equivalent to Earth right down to their "native" intelligent inhabitants being humans