So pretty much, the story of the Body Collectors started with a 2005 haunted house called Body Collectors. The year had a huge backstory about a country that was run on human sacrifice. The inhabitants of the country, called Terra Cruentus, would use some of the parts to make the Terra Blade, used for sacrificing the Terra Queen on Halloween night every 15 years. The Body Collectors, were the ones who would process the bodies and sort the parts. They would use these parts to make "art" and tools. The house that year involved guests entering their industrial factor located in Terra Cruentus. Inside, guests could see them mutilating bodies to salvage their parts.
HHN 2008 expanded on this, in a prequel house and scarezone titled Body Collectors: Collections of the Past and the Streets of Blood respectively. The stories behind these two, were that the Body Collectors were using cover ups of mass murder and plague to hide their "body collecting" (taking bodies to harvest their organs). The event featured in this house and scarezone was the Jack the Ripper murders. The Body Collectors would take the bodies and blames it on Jack the Ripper. Their assistants also appeared in this house and scarezone, as people who would help the Body Collectors collect.
Now, fast-forward to HHN25 and the lore of the Body Collectors was expanded even further. Body Collectors: Recollections was a haunted house tie-in with another famed HHN franchise, Psycho Scareapy. The lore behind that house was that Shadybrook Asylum went through a blizzard, and that the Body Collectors, finding a perfect opportunity as everyone was trapped inside, started to collect bodies. They enlisted the help of some of the inmates, and together, they all started to kill people in various ways.
Also, the Body Collectors were based on The Gentlemen from A Buffy the Vampire Slayer episode called Hush.
That's all I know. Hope, this helped!