So this is a question I've been wondering since TESV as well, and while I've read at least 90% of the books in this game and ESO (rather, I read every book I can find), I've never really found a definitive answer to what their deal is.
to clarify, the question I have is "why are they hostile to you?" (the player, and/or followers)
I mean game-wise, I get it. ZOS might want some low-power mook that isn't a critter or a Falmer, but I want to know if there's a lore-wise explanation for how the victims become thralls like this.
the term "Thrall" makes me think that it's similar to a vampire thrall insomuch that there's no saving them (which would make sense given that these NPCs are hostile rather than neutral). But behaviour-wise, they act more like slaves when we see them, usually just doing manual labour here or there or whatever.
obviously it makes sense for Falmer to be hostile due to their "hatred of the living" or of those on the surface or whatever, but even with as xenophobic as these Western Nords are, they don't attack outsiders on sight. To me, that in turn means that something had to have happened to their minds/the way they think. Given the Falmer, I imagine something alchemical, but at the same time I could come up with 1001 questions and theoretical answers and unless there's a puzzle on some quest or something, I'd rather stick to facts over speculation.
I understand that Falmer are still roughly in unknown status at this point in time as well, but given that blackreach thralls exist in TESV when the Falmer are a lore more widely known and there's no answer for -why- they exhibit this hostile behaviour, much less the origins/cause of it... I'm, asking the question.
TL;DR—basically, why do these ordinary-looking (albeit raggedly-dressed) people want to kill us, and what's the story behind what happened to make them that way? does anyone have any lore-related answers?
Shortly after the formation of the Ebonheart Pact, a Nord woman was given a tour of the Tribunal Temple. When later asked about the experience, she seemed upset. Suffice to say, the Dunmer were not pleased to hear this, and thus they inquired further.
"Well," the Nord frowned, "the priests were very angry and unwelcoming. They kept shouting things at me like "you can't drink that mead in here!" and "somebody stop her, she's running naked!" and "we can't catch her; she's covered in grease!""