Suppose my opponent has a Butcher of Malakir and an Extractor Demon on the field, and they have no other creatures, and I have a Nuisance Engine and let's say, oh, eight 0/1 colorless Pest artifact creature tokens on the field, and then I cast Barter in Blood, which says that "each player sacrifices two creatures".
Nuisance engine is tapped and has no passive abilities, so it's irrelevant here, except that it created the Pests. Butcher of Malakir says that "Whenever Butcher of Malakir or another creature you control dies, each opponent sacrifices a creature," and Extractor Demon says "Whenever another creature leaves the battlefield, you may have target player put the top two cards of their library into their graveyard." Extractor Demon also has an Unearth ability, but I think that's also irrelevant here.
Now, I've read the card texts, I've read the rulings, but there's a lot going on here, so I just wanted to make sure I got this right. When I cast Barter in Blood, it is not countered and is allowed to resolve, I choose two of my Pests to sacrifice, and my opponent chooses to sacrifice Butcher of Malakir and Extractor Demon. So then all the creatures are sacrificed, and Butcher of Malakir's ability goes on the stack twice, because Butcher and Extractor Demon died, and Extractor Demon's ability goes on the stack three times (with my opponent targeting my library), because Butcher and two of my Pests left the battlefield, and these all go on the stack simultaneously, so my opponent chooses to put both of the Butcher abilities on the stack first, then all of the Extractor Demon abilities. The Extractor Demon abilities all resolve, one after another, and result in a total of six cards being milled from my library, and then Butcher's abilities resolve, and I have to sacrifice a Pest, twice. And since Butcher of Malakir is no longer on the field, there are no more active effects, or passive abilities, or whatever they're called, and that's the end of that.
tl;dr: So if I got this right, my opponent sacrifices both of his creatures, I wind up sacrificing a total of four Pests over the course of this mess, and losing six cards off the top of my library, and have four Pests remaining, as well as my tapped Nuisance Engine. Did I get this right? This was a lot to process.